Added BOARD_[CONNECTED_]ITEM::IsConnected() for checking if a BOARD_ITEM is BOARD_CONNECTED_ITEM as well.

Added NETINFO_MAPPING, to ease saving nets with consecutive net codes (without modifying the net codes during the run time).
Now, nets are saved with consecutive net codes (both modern & legacy plugins).
Zones are saved together with their nets, without depending on the fact if there are any pads with such net. Therefore validation of zone net names was removed (pcbnew/class_board.cpp).

Performed tests:
- Changed a pad's net name from empty to existent - ok, name was changed.
- Changed a pad's net name from empty to nonexistent - ok, error message is displayed, net name stays empty.
- Changed a pad's net name from existent to empty - ok, net name became empty
- Changed a pad's net name from existent to nonexistent - ok, error message is displayed, net name is not changed.

- Drawn a zone that belongs to a net, then modified schematics so the net does not exist anymore. After reloading the net list, all pads/tracks are updated. Zones still belongs to the net that does not exist in the schematic (but still exists in .kicad_pcb file). After running DRC, the zone becomes not filled.
- Undo & redo affects assignment of a polygon to a specific net (you may change net of a polygon, refill it and undo/redo the changes).

- KiCad s-expr & legacy, Eagle, P-CAD boards seem to load without any problem (they also contain correct net names assigned to the appropriate pads). All types of board file formats were loaded, then saved in sexpr format and reopened with a KiCad built from the master branch (without my modifications).
- A few boards were also saved using the legacy format and were opened with the master KiCad without any issues.

- Change a net name for a pad, restore with undo/redo - ok
- Remove everything, restore with undo - ok
- Remove everything, reload netlist - ok

Differences observed between files saved by the master branch KiCad and this one:
- list of nets are not saved in any particular order, so net codes may differ
- the default net class does not contain the unconnected net

Add EDA_BASE_FRAME::Show() and call SaveSettings() only from there for all derived wxFrames.  Remove calls to SaveSettings() elsewhere.

github_plugin.cpp: in error message relative to"cannot get  zip archive...", line 496, make the sentence relative to http GET command not translatable
inside the full translated message.

Remove g_LibraryNames and all supporting infrastructure from pcbnew.  FP_LIB_TABLE is now local to major wxFrames.

Pcbnew: fix bug #1272714 . Code cleanup: use 2 different enums for pad shapes and pad drill shapes. Remove never used pad shape.

Increase boost download TIMEOUT to 10 minutes, offer a LOG_DOWNLOAD option for capturing download problems.

Fix not working + and - keys in layers manager.
Fix typo errors and modify some messages to make translator's life more easy.

NETINFO_ITEMs are not stored in a vector anymore, instead they are held in a unordered_map. Now, the net codes may be not consecutive. There is another way for assigning net codes (using a static int that holds a possible empty net code and a function that makes sure it is not used [getFreeNetCode()]).
Removed some unused fields (NETINFO_ITEM::m_NbNodes, m_NbLink, m_NbNoconn, m_Flag).

BOARD_CONNECTED_ITEMs do not store net code anymore (m_NetCode field), instead net info is stored using a pointer to NETINFO_ITEM.
GetNet() refers to the net code stored in the NETINFO_ITEM. SetNet() finds an appropriate NETINFO_ITEM and uses it.
Removing GetNet() & SetNet() (and the whole net code idea) requires too many changes in the code (~250 references to the mentioned functions).
BOARD_CONNECTED_ITEMs by default get a pointer to NETINFO_ITEM that stores unconnected items. This requires for all BOARD_CONNECTED_ITEMs to have a parent (so BOARD* is accessible). The only orphaned item is BOARD_DESIGN_SETTINGS::m_Pad_Master, but it does not cause any issues so far.
Items that do not have access to a BOARD (do not have set parents) and therefore cannot get net assigned, by default get const static NETINFO_LIST::ORPHANED.

Performed tests:
- loaded .kicad_pcb, KiCad legacy board, Eagle 6.0 board, P-CAD board - all ok
- load a simple project, reload netlist after changing connections in eeschema - ok
- save & reload a board - ok, but still contain empty nets
- remove everything, restore with undo - ok
- remove everything, reload netlist - ok
- changing net names (all possibilites: empty->existing, empty->not existing, existing->empty, existing->not existing) - all ok
- zones: when net is changed to a net that does not have any nodes besides the zone itself, it does not get filled

- ZONE_CONTAINER::m_Netname field
- ZONE_CONTAINER::SetNetName()
- ZONE_CONTAINER::SetNet() [it uses the one in BOARD_CONNECTED_ITEM]
- ZONE_CONTAINER::GetNetName() [instead BOARD_CONNECTED_ITEM::GetNetname is used]
- ZONE_CONTAINER::SetNetNameFromNetCode()

Performed tests:
- Drawn a zone that belongs to a net, then modified schematics so the net does not exist anymore. After reloading the net list, all pads/tracks are updated. Zones still belongs to the net that does not exist in the schematic (but still exists in .kicad_pcb file). After running DRC, the zone becomes not filled.
- Undo & redo affects assignment of a polygon to a specific net (you may change net of a polygon, refill it and undo/redo the changes).

D_PAD::GetNetname() and D_PAD::GetShortNetname() were moved to BOARD_CONNECTED_ITEM. Now they use the net name stored in NETINFO_ITEM.
Moved some one-line functions from class_board_connected_item.cpp to class_board_connected_item.h.
Added a copyright notice, moved Doxygen comments from class_board_connected_item.cpp to class_board_connected_item.h.

I have some doubts if changes introduced pcbnew/dialogs/dialog_pad_properties.cpp do not break anything, but I could not find a test case that breaks the pcbnew.

Performed tests:
- changed pad's net name from empty to existent - ok, name was changed
- changed pad's net name from empty to nonexistent - ok, error message is displayed, net name stays empty
- changed pad's net name from existent to empty - ok, net name became empty
- changed pad's net name from existent to nonexistent - ok, error message is displayed, net name is not changed
- (re)reading netlists, including net changes - fine, changes are applied, but empty nets are still kept
- loaded pcbnew/pcad2kicadpcb_plugin/examples/CK1202_V1.pcb to test P-CAD import plugin - ok, net names are correct
- imported an Eagle 6.0 board (Arduino Uno; http://arduino.cc/en/uploads/Main/arduino_Uno_Rev3-02-TH.zip) then saved in .kicad_pcb format and reloaded - ok, net names are correct
- saved demos/video/video.kicad_pcb in legacy format and then loaded it again - ok, net names are correct

Changed the way of looking up NETINFO_ITEM using net names (using boost::unordered_map). Added a hash function (wxString) for that. Introduced NETINFO_ITEM::GetNetItem( wxString ). BOARD::FindNet() uses the map.
Net codes are updated upon net list update. (BOARD::ReplaceNetlist())
Added in some places (mostly class_board.cpp) pad->SetNet() calls to synchronize net codes.
On creation of NETINFO_LIST, the first NETINFO_ITEM is added (the unconnected items net).
Removed COMPONENT_NET::m_netNumber, as it was not used anywhere.
Added an assert to D_PAD::GetNetname(), checking if net code and net name is consistent for unconnected pads. Added an assert for NETINFO_LIST::AppendNet() to assure that appended nets are unique.

It seems that at this point:
- Updating net lists works fine. The only difference between the file ouput is that after changes it contains empty nets as well.
- Nets are not saved in the lexical order. Still, net names and net codes are properly assigned to all items in the .kicad_pcb file. It is going to be addressed in the next commit. I believe it should not create any problems, as pads are sorted by their net names anyway (NETINFO_LIST::buildPadsFullList())

Performed tests:
- Created a blank PCB, saved as pic_programmer.kicad_pcb (from demos folder). Updated net lists. .kicad_pcb file (comparing to the results from master branch) differ with net order (as mentioned before), net codes and timestamps.
- Removed some of components from the above .kicad_pcb file and updated net lists. Modules reappeared. .kicad_pcb file differs in the same way as described above.
- Trying to change a pad net name (via properties dialog) results in assert being fired. It is done on purpose (as there is a call to GetNetname() and net name and net code do not match). This will not happen after the next commit.
- Prepared a simple project (starting with schematics). Imported net list, changed schematic, reimported net list - changes are applied.
- Eagle & KiCad legacy boards seem to load without any problem.