Revision 191620

Resolving issue: MNG-493

o Changed getExecutionsAsMap() in Plugin (within maven.mdo) to throw an IllegalStateException if two PluginExecutions are detected with the same id. This method should always be called during inheritance assembly...

o Changed PluginExecution's id field to have a defaultValue of 'default' within maven.mdo.

NOTE: It may be necessary to double-check the id-collision within MavenProjectBuilder, in case there is no inheritance of plugins...I'll add this, and follow this commit momentarily.