Add top margin to site footer

Because we're using a sticky footer technique, also need to compensate for size of top margin in the bottom margin of the body tag.

**Before**

![browse_exhibit___default_exhibit_-_blacklight](https://cloud.githubusercontent.com/assets/101482/5075398/6b4d3756-6e47-11e4-9428-02970452493c.png)

**After**

![browse_exhibit___default_exhibit_-_blacklight](https://cloud.githubusercontent.com/assets/101482/5075448/ae358500-6e47-11e4-84b8-4a050e42bdfb.png)