Anchor links for the site's headers which have ID's

For every header on the page inside the main content section which has an ID, it is assigned a clickable anchor that appears when the header is hovered over:

![screen shot 2014-05-07 at 4 41 42 pm](https://cloud.githubusercontent.com/assets/237985/2908562/24217006-d629-11e3-8dd0-2b3e8db62277.png)

Fixes #2330