Style the Footer for the landing page

* Add footer partial
* Add FontAwesome support
* Add copy for footer
* Add thoughtbot logo

![middleman-footer](https://cloud.githubusercontent.com/assets/816402/4909369/ca538cde-6472-11e4-9a0f-b2857c0d40e0.png)

Eventually, these font-weights could be variables relating to the what fonts are available (light, normal, semibold, bold, etc)