Feature markdown preview

Markdown preview for discussions and comments.  Pretty cool!

![screen shot 2014-02-27 at 12 54 27 pm](https://f.cloud.github.com/assets/657707/2286351/af6c12bc-9fe0-11e3-97f6-463b831fce3f.png)

![screen shot 2014-02-27 at 12 54 42 pm](https://f.cloud.github.com/assets/657707/2286352/b21c4892-9fe0-11e3-85de-1a992ae9e0d0.png)