Style the hero portion of Middleman landing page

* Add `Fira` fonts to directory
* Add new color variables for Middleman branding
* Install Bourbon, Neat, and set CSS to very basic
* Edit hero content in copy
* Update hero with relevant blocks of content before CSS style of terminal
* Add browser using code

![middleman-hero](https://cloud.githubusercontent.com/assets/816402/4891784/a670dc4e-63ab-11e4-8bfa-e5bd1338e698.png)

Should we move these `site-*` classes to a `_nav.scss` partial?