Generate scaffold for Cart
Cart Retrieval and Creation

Create a CurrentCart concern than can be used by all controllers to
deal with the Cart. This handles the happy path of a cart already existing
when a user needs it, and the unhappy path of there not being a cart when
for the user when he/she needs one.

The set_cart method retrieves the appropriate cart
from the database, using the session to do so. If the session does not
contain the information about the cart, Then a new cart is created and
the session is assigned information about that new cart.
add line-item scaffold
create relationship between product and line items.
add button to add products to cart
create relationship between product and cart through line_item
test line_item creation and relationship with cart and product
add model relationship between Cart and LineItems
display cart in cart show page
Merge pull request #3 from arcin/cart

Cart