Clean up Text Cell display

Closes #72 and #162. 

Text cells now have a smaller font, reduced padding, and lighter border.

![screen shot 2013-12-08 at 12 23 00 pm](https://f.cloud.github.com/assets/212941/1701205/d02e72c0-6047-11e3-9880-0efa523a7ffe.png)