Doc layout quest

This commit presents a fixed navigation and header structure to mapbox.js documentation. It also introduces Swiftype to take care of search. One other thing to note is that the landing page is now the first page of the API. Here's an animated gif and screenshots:

### Splash + API
![splash](https://f.cloud.github.com/assets/61150/2297996/b0e3e46e-a0b5-11e3-9d77-a48799191c6f.gif)

### Examples
![examples](https://f.cloud.github.com/assets/61150/2297994/b0d8ecee-a0b5-11e3-8ab1-09c50487996d.png)

### Plugins
![plugins](https://f.cloud.github.com/assets/61150/2297995/b0ddc55c-a0b5-11e3-8b1f-4486cf05b0b3.png)

cc/ @yhahn @lxbarth @tmcw