Style the Footer for the landing page

* Add footer partial
* Add FontAwesome support
* Add copy for footer
* Add thoughtbot logo
* Add modules directory to organize stylesheets
Enable middleman-autoprefixer to create vendor prefixes

* Add `middleman-autoprefixer` to `Gemfile`
* Configure `middleman-autoprefixer` in `config.rb`
* Disable `bourbon` prefixes in base settings

https://trello.com/c/Ex4aRfuZ
Add nav partial and move nav markup to partial
Rename navigation partials to be more semantic
Change href links to use link_to method
Add contribute translation
Rename classes in nav
Change hover color of cta in nav
Change contribute pathway