BS3: Navbar overhaul

![screen shot 2013-08-12 at 9 39 27 pm](https://f.cloud.github.com/assets/98681/952548/7dacd8e2-03d2-11e3-85f2-62f062b7afe1.png)

<img src="https://f.cloud.github.com/assets/98681/952550/8d4d3332-03d2-11e3-96ae-f50c723624b3.png" width="392">
<img src="https://f.cloud.github.com/assets/98681/952551/8d92421a-03d2-11e3-8c76-dd46a5c2d082.png" width="392">

**This overhauls our navbars to address a few items:**

* Navbars are now responsive by default (fits better with mobile first approach)
* Navbars now require `.nav-header` to group `.navbar-brand` and `.navbar-toggle` for proper mobile display in conjunction with the collapsed content.
* Renamed `.nav-collapse` to `.navbar-collapse` for a more consistent naming convention.
* Refactored dropdown menus to use the old 2.x approach (no longer absolutely positioned, but rather as navbar nav links).
* Simplified examples in docs to only highlight content not displayed in the first navbar example.

It can add some heft to an otherwise lightweight navbar, but I think all these improvements help the component and make it more durable in the long run. Still needs some finessing, but it's close I think.