Docs navigation

This is a scaled back commit as reflect in comments [here](https://github.com/mapbox/mapbox.js/pull/678)
Improved navigation flow with fixed navigation as you scroll down the page.

![screen shot 2014-02-18 at 10 54 49 am](https://f.cloud.github.com/assets/61150/2196841/0fb3e678-98b5-11e3-88ce-08a5f6ef0ee2.png)

cc/ @lxbarth @tmcw