Tell Maven's exec:java to execute our main class by default

It is quite convenient to be able to tell Maven to execute the plugin
(thanks to the main() method) by calling

    mvn package exec:java
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

pom.xml

@@ -26,6 +26,25 @@
 		</dependency>
 	</dependencies>
 
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>org.codehaus.mojo</groupId>
+				<artifactId>exec-maven-plugin</artifactId>
+				<executions>
+					<execution>
+						<goals>
+							<goal>java</goal>
+						</goals>
+					</execution>
+				</executions>
+				<configuration>
+					<mainClass>Process_Pixels</mainClass>
+				</configuration>
+			</plugin>
+		</plugins>
+	</build>
+
 	<developers>
 		<developer>
 			<id>dscho</id>