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)

I would recommend optimizing this
```
npm install -g svgo
svgo --pretty source/images/thoughtbot-logo.svg
```