Add GitHub mark to GitHub links

This was requested in #95.

Adds a little 10x10px GitHub mark to every GitHub link in the main
frame.

![screen shot 2013-05-06 at 5 35 48 am](https://f.cloud.github.com/assets/24421/465607/74b18a58-b639-11e2-9366-8328fb870c27.png)