Highlight current section

First iteration of highlighting the main nav link for the section the current page is within.

![screen shot 2014-10-07 at 1 55 51 pm](https://cloud.githubusercontent.com/assets/1239550/4536763/88569c10-4dcd-11e4-8969-91a87d8c1542.png)
![screen shot 2014-10-07 at 1 55 28 pm](https://cloud.githubusercontent.com/assets/1239550/4536764/885d3f8e-4dcd-11e4-8cd4-1ab45cdb7618.png)
![screen shot 2014-10-07 at 1 55 21 pm](https://cloud.githubusercontent.com/assets/1239550/4536765/885f1fde-4dcd-11e4-99cb-deae3fc12783.png)

closes #598

Also removes 'help' from the top nav (closes #643)