Place top nav over everything else

The responsive layout on the home page uses `z-index` to correctly layer
the images. This causes some of the drop down items in the top nav to
render behind these images, making them inaccessible on the home page.
Giving the top nav a higher `z-index` fixes this.

Closes #22
Merge pull request #23 from PyGotham/drop-down-z-index

Place top nav over everything else