First draft of source docs

Quick and dirty attempt at source documentation, plus some usability tweaks that came up in the process:
![screen shot 2014-03-31 at 7 16 17 pm](https://cloud.githubusercontent.com/assets/108094/2574090/7f8f8e3a-b92a-11e3-8f71-a54f218c58a3.png)

Ideally this gets merged in sooner than later, and improved as we go.