Removed null checking for Item's container (container can be null)
Refactor: Moved StorageUnit's add(Item) method into ProductContainer
Fixed ProductContainer's add(Item) method so that the Item is added to
the correct ProductContainer that contains the Item's Product.
Merge branch 'master' of https://github.com/cletusw/hit.git

Conflicts:
	src/model/ConcreteProductContainerManager.java
	src/model/StorageUnit.java
	test-src/model/ProductContainerTest.java