Group subtopics by topics on index.

https://www.pivotaltracker.com/story/show/77978874

Before:
![screenshot 2014-09-05 10 28 14](https://cloud.githubusercontent.com/assets/109225/4162746/ff2dd752-34de-11e4-9d03-ba9b8537a1cf.png)

After:
![screenshot 2014-09-05 10 28 41](https://cloud.githubusercontent.com/assets/109225/4162748/10bf29bc-34df-11e4-83fe-7057a495a896.png)