Minor UML fixes.
Merge pull request #7 from jr-news/seth

Minor UML fixes.
Updated class diagram.
Merge pull request #8 from jr-news/seth

Updated class diagram.
Implement Product
Further Product implementation.
Updated Javadoc comments in Product.java.
Generated JUnit test case for Product.
Implemented test methods for Product.equals() and Product constructor.
Updated ProductList method stubs.
Implemented ProductList methods.
Created and implemented Cart.
Created and implemented Inventory.
Wrote and edited javadoc comments for Inventory, Cart, and ProductList.
Implemented the rest of the test methods for Product.
Merge remote-tracking branch 'origin/seth'
Generated JUnit test class for ProductList
Generated JUnit test class for Cart.
Generated JUnit test class for Inventory.
Deleted InventoryManager.
Updated PaymentValidator