Footer Background Color Change

# Changes
- sets the global footer background color to black
- increases `.social.tablet` width to fill space left by do something logo

![footer](https://f.cloud.github.com/assets/1479130/2422890/7e6bd31e-ab93-11e3-9c4a-a1ed977f51c8.png)

Fixes DoSomething/dosomething#1197