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.