add github button

Regular
![screenshot 2013-11-09 08 43 40](https://f.cloud.github.com/assets/2801090/1506762/6b403f42-4945-11e3-8705-854f15d986cf.png)

On hover:
![screenshot 2013-11-09 08 43 48](https://f.cloud.github.com/assets/2801090/1506763/6d4e9004-4945-11e3-9719-f128fdbeeab0.png)