Put in a generic Menu type. Trying to figure out how to proceed.
Merge branch 'master' into flexible_menu

Conflicts:
	src/building/Airport.java
	src/building/Base.java
	src/building/Building.java
	src/building/Port.java
	src/unit/Unit.java
Added some more structure around Pieces
Finished implementing the flexible menu system.
Current limitiations: you cannot go to the previous menu when you select
a Piece's action.
Clarification: there's three constructors for Menu. This determines (in
order):
	1. Top menu (children are all the Pieces)
	2. Piece menu (children are all the Piece's possible actions)
	3. Action menu (children is null)