Adding (mostly) general navigation styles

Includes:
 - examples of how to use the +icon mixin for loading social icon links
 - easy way to add horizontal separators to menu items
 - example of how to use the menu toggle button to hide a menu in a
mobile view (applied to .menu-main-menu-container)
Switching to compressed CSS output

Save around 35% on file size
Also includes a great technique to prevent WP theme comment header from
being stripped, which is to start the comment with a bang: `/*!` (via
[Chris
Coyier](http://css-tricks.com/compass-compiling-and-wordpress-themes/))