Modified DatabaseModel, ShoppingList, removed DatabaseModelHelper

author: Amy Zhang

Modified the DatabaseModel methods to use ThePantryContract class. Also
changed the methods to use SQLiteQueryBuilder.

ShoppingList methods now use the DatabaseModel class, code is currently
commented out to allow testing for display purposes. Will have to write
test code that test database functionality as well as displays.

Deleted the DatabaseModelHelper class as we no longer need this.