Improve Help navigation UI

This PR brings the Help navigation in line with the rest of the sidebar navigation UI. 

Before:
![helpnavbefore](https://f.cloud.github.com/assets/2394772/1647027/d3745ea0-592f-11e3-927a-bd6136029094.png)

After:
![helpnavafter](https://f.cloud.github.com/assets/2394772/1647028/d7eec01a-592f-11e3-97b3-8aef4e475c58.png)