Let .navbar-wrapper flow in Carousel example

A `.navbar-wrapper` followed by a `.carousel` works well in this example, but when the navbar is not followed by a carousel, content flows underneath the navbar because it is absolutely positioned.

Letting .navbar-wrapper be part of the flow and moving the negative margin to the .carousel to bring it underneath the navbar makes these example styles suitable for pages without carousels.

*Previously committed to Bootstrap core: https://github.com/twitter/bootstrap/pull/8474*

### Current styles, no carousel:
![screen shot 2013-07-18 at 2 51 46 pm](https://f.cloud.github.com/assets/29612/822691/452f7f60-eff5-11e2-8f4f-6768498e79ae.png)
![screen shot 2013-07-18 at 2 51 41 pm](https://f.cloud.github.com/assets/29612/822693/4a4d4c16-eff5-11e2-8b0d-0fac6fa16fed.png)

### After changes, no carousel:
![screen shot 2013-07-18 at 2 51 16 pm](https://f.cloud.github.com/assets/29612/822699/5d6731c2-eff5-11e2-81c5-f1bd65918d25.png)
![screen shot 2013-07-18 at 2 51 22 pm](https://f.cloud.github.com/assets/29612/822700/5f8a4692-eff5-11e2-9ef8-d81024f78423.png)