add headings to sidebar

Adds headings to sidebars:

![screen shot 2014-03-27 at 6 41 14 pm](https://cloud.githubusercontent.com/assets/108094/2544470/ed25d616-b600-11e3-80a7-55d430b3659c.png)

![screen shot 2014-03-27 at 6 40 18 pm](https://cloud.githubusercontent.com/assets/108094/2544468/e5aa9192-b600-11e3-8fe0-b557da9c8a02.png)

Not sure about 'active sources' and 'available sources' for heading  txt...

Addresses issues from #244