Enhc full bg featured img

This is a "featured" project image that spans from top of the page
til the next section.

Uses a new background image class and user applies a background url
along with a height inline.

Not 100% confident in my choice to use z-indexing on this solution, but
am also not 100% sure on how this bg image and other will be used
across the site.

Other changes:
- Setup z-index variables to avoid collisions.

![screen shot 2014-10-27 at 4 44 23 pm](https://cloud.githubusercontent.com/assets/1701077/4813715/e6a875a0-5eca-11e4-918a-f1f8f8e8da6b.png)

Should this be the style for the `section` tag below (and remove the `img`)?