Support markdown comments

Parse and render comment bodies as markdown. 
Note that since markdown can include HTML, this does not affect the existing support for HTML tags in comments.

Tested with GISTUB_AUTO_LINK set to both true and false.

* GISTUB_AUTO_LINK == false
  * Normal URLs are NOT auto-linked, but Markdown-formatted links appear as links
* GISTUB_AUTO_LINK == true
  * Normal URLs ARE auto-linked, and Markdown-formatted links also appear as links

Screen shot (with GISTUB_AUTO_LINK == false):

![screen shot 2014-04-21 at 16 00 18](https://cloud.githubusercontent.com/assets/106760/2752910/6847bd5c-c923-11e3-9162-3d680a76bdc9.png)