Fixing footer formatting that got broken in recent work

Somehow the CSS and formatting of the footer got messed up in recent work, with multiple borders that are offset and vertical spacing that is too abundant. This cleans that up and goes a bit further with cleaning up the multiple columns and the descriptive text.

Here is currently deployed nbviewer footer:

![screen shot 2014-04-22 at 8 46 07 pm](https://cloud.githubusercontent.com/assets/27600/2773418/e132f4ee-ca99-11e3-94d5-39a46636a491.png)

And with this PR:

![screen shot 2014-04-22 at 8 45 02 pm](https://cloud.githubusercontent.com/assets/27600/2773419/e5d5830e-ca99-11e3-83b8-5a58bfab67e6.png)

![screen shot 2014-04-22 at 8 45 17 pm](https://cloud.githubusercontent.com/assets/27600/2773420/e8851c18-ca99-11e3-9cf7-ce590453ef00.png)