docs(demo): added link to src files in github

Added a link that points to the src files in github:
![image](https://cloud.githubusercontent.com/assets/706039/2778193/44e8aa42-caef-11e3-8428-98e583865171.png)

For example, the link for 'configs' takes you to https://github.com/rackerlabs/encore-ui/tree/master/src/configs/

Note: I didn't spend much time on the styles for this, since I'll be working next on the demo page redesign.