Highlight active nav section

This gives the active nav section the same color as the hover effect. The active nav section also no longer functions as a link.

Subsections do not have the same highlight. Should they?

Tested in Chrome and Firefox.

![screen shot 2014-04-06 at 11 25 25](https://cloud.githubusercontent.com/assets/6269/2624921/748d15a6-bd6d-11e3-8181-915a634c7a65.png)