Updated UndoManager to return Commands for use by the View.
Implemented Undo / Redo for AddItems command

Added undoManage method for ItemManager and corresponding wrapper method
for ProductContainer.remove(Item, ItemManager, boolean)
Created AddProduct class implementing Command interface.
Merge branch 'master' of https://github.com/cletusw/hit.git

Conflicts:
	src/model/Product.java
	src/model/ProductContainer.java