Provide highlight style in documentation navigation. Closes #21

Mobile:
![screen shot 2014-09-22 at 17 15 49](https://cloud.githubusercontent.com/assets/580859/4359663/d149aabc-4273-11e4-97b6-e55cc2554262.png)

Desktop:
![screen shot 2014-09-22 at 17 16 05](https://cloud.githubusercontent.com/assets/580859/4359665/d78945fe-4273-11e4-99fb-81a4797fb6fe.png)