Add bottom margin to hr elements

This gives some more consistent spacing, and looks better.

Before:

![screen shot 2013-05-08 at 5 08 26 pm](https://f.cloud.github.com/assets/99231/480059/a4405d62-b823-11e2-9c7c-4a9da94c2e18.png)

After:

![screen shot 2013-05-08 at 5 08 48 pm](https://f.cloud.github.com/assets/99231/480058/a43a6092-b823-11e2-8ef2-3b2869f05e2f.png)