Version 0.0.6 Cart Functionality

1. Cart functionality
	create cart if none exits already
	created add to cart functionality
Version 0.0.7 Added Smarter Cart

1. Added quantity column to line_items with default value 1
2. Migrated existing data into the new table format
3. Added Exception Handling to Cart
4. Removed cart parameter from the permitted list of line item
5. Added empty cart functionality
6. Added total price methods to both line item and cart and updated corresponding views