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.

### API
![screen shot 2014-02-18 at 10 55 54 am](https://f.cloud.github.com/assets/61150/2196854/32032d06-98b5-11e3-8276-50c63b9e9339.png)

### Examples
![screen shot 2014-02-18 at 10 56 38 am](https://f.cloud.github.com/assets/61150/2196865/52d41ec8-98b5-11e3-97e6-6b6c13a9aeca.png)

### Plugins
![screen shot 2014-02-18 at 10 56 50 am](https://f.cloud.github.com/assets/61150/2196866/59051022-98b5-11e3-85d9-f88be92d42f8.png)

cc/ @lxbarth @tmcw