Add ajax support for cart

* Moved shopping cart into the sidebar
* Used remote:true to invoke the LineItemsController.create() action
  using ajax
* Added a highlight effect, using the jQuery-UI library.
* Wrote a helper method that hides the cart when it is empty and used
  jQuery to reveal it when an item is added.
* Added a bit of CoffeeScript in order to cause items to be added to
  the cart when an image is clicked.