Code button

Adds a github button if `code` is defined in the post template. Expects a string in the form `<username>/<repo name>`.

It set it in the interview post to confirm it worked:
![screenshot 2013-11-09 11 45 37](https://f.cloud.github.com/assets/2801090/1507013/63cfd330-495e-11e3-803f-b02210c5fa72.png)