User Story 5, Cart Creation

[memcmahon]

As a visitor
When I visit an item's show page from the items index
I see a link or button to add this item to my cart
And I click this link or button
I am returned to the item index page
I see a flash message indicating the item has been added to my cart
The cart indicator in the navigation bar increments my cart count

DO THESE STORIES FIRST:

• User Story 4, Cart Indicator

Checklist:

• base tests are written
• base code is written to pass tests
• edge case ("sad path") tests written (if required)
• edge case code written (if required)
• code is tested in development
• code is reviewed
• branch is merged
• all tests still pass

child of #32

child of #37

depends on #31