Add Shopping Cart bare-bones API

Shopping Cart service now available and has the following API methods - not yet implemented, they currently return empty JSON arrays
- GET {HOST}/cart - Returns the shopping cart contents
- PUT {HOST}/cart/{PRODUCT_ID} - Add a product to the cart
- PUT {HOST}/cart/{PRODUCT_ID}/{QUANTITY} - Add a product to the cart with the given quantity
- DELETE {HOST}/cart - Empty all the shopping cart contents
- DELETE {HOST}/cart/{PRODUCT_ID} - Remove a product from the shopping cart
- DELETE {HOST}/cart/{PRODUCT_ID}/{QUANTITY} - Remove a quantity of products from the cart

Other changes:
- Fix Guice JPA integration. Now works as designed, performs transactions on a per-request basis
- Update user/register method to add the user to the CUSTOMER role
- Update user methods login and register to use HTTP methods POST and PUT respectively as opposed to GET