Adds brand_html_left and brand_html_right options to NavBar helper.

Enables the user to pass in optional html to appear before or after the brand.

![brand_html_right](https://f.cloud.github.com/assets/1134807/1647793/f889649c-5952-11e3-9abf-8af6b35e8b33.png)
![brand_html_right_collapsed](https://f.cloud.github.com/assets/1134807/1647813/ff36994e-5953-11e3-85be-2572f10a6e7f.png)

A similar result can't be achieved in the nav_bar block because all of that gets wrapped into the "nav-collapsed collapsed" div, where it breaks away from the brand when collapsed.