Varun/cart view

This pull request refactors the way that all cart logic is implemented. Instead of mangling together a bunch of React components, I added backbone.js to handle the state of the cart object. Now the components are updated whenever there is a change on the backbone model.

Fixes #91 

"If no context is specified, all of the versions of the callback with different contexts will be removed. If no callback is specified, all callbacks for the event will be removed."

If you have other components listening on cart, I think this will remove their listeners as well

Instead, maybe do something like in https://gist.github.com/ssorallen/7883081 where you define a `this._boundForceUpdate()` function on the component?