Ajax cart updates

- Prevent non-numerical characters from becoming `Nan` in the quantity selectors
- Added a remove button to the cart under the title
- Minor fixes to the checkout CSS (a button was invisible)

With the latest PR that converted the cart layout to a table, the max-height CSS animation in the ajax cart now doesn't work. You cannot animate the height of a table row without JS. Example of height not animating below.

![skdw8azijl](https://cloud.githubusercontent.com/assets/1730309/4002744/c74c153a-296f-11e4-8fe8-5d11a56fa5ee.gif)


#### Options
- Revert to div-based cart layout
- Add wrapping div around all content in the table cells and animate its height rather than the row
- Remove the animation all together

cc/ @Shopify/fed