Symanticize headers

Fixes #18.

This moves all in-document headers to H3s or less (from h1s, h2s, and h3s), and moves navigation to a UL, rather than being dual H3s.

The navigation links are now automatically generated (separating content from presentation) and floated right/left for next previous.

## Before

![screen shot 2014-09-16 at 10 31 53 am](https://cloud.githubusercontent.com/assets/282759/4288706/3a6d16c0-3dae-11e4-973c-7367fc6ea7a5.png)

## After

![screen shot 2014-09-16 at 10 27 41 am](https://cloud.githubusercontent.com/assets/282759/4288684/0e172a48-3dae-11e4-9f7a-e8786ef9a338.png)