Move heading anchors 50px up to avoid nav bar

Fixes #447.

We do this by moving the actual anchored element up in the page without moving the actual text. (Apple uses a similar trick in their framed docs.) Now this looks a bit sillier on smaller screens but it's better overall.

Before:

![image](https://f.cloud.github.com/assets/6820/1416683/7846c07c-3f62-11e3-8898-074fb4d03ac6.png)

After:

![image](https://f.cloud.github.com/assets/6820/1416684/83cb3e00-3f62-11e3-9e9c-a784bfd186d9.png)