Add navigation style consistent with main site

* Added variables file
* Re-created Middleman global_nav using code from main site
* Haven't touched main content grid - need to align with header content

![middleman directory](https://cloud.githubusercontent.com/assets/5547897/5112700/696f38e2-6fde-11e4-98a6-162b57db2ce7.png)
Same with `docs` so it appears here as it does on `middleman-guides`