Sass: Navigation

Needs mobile testing
Add jQuery as a Bower dependency
Add Sass source map
Add Codekit config file
Part 1: Switch to using rem units with no px fallback

Before the ‘rem’ mixin took care of both rem and px units. But the
browser support for rem is quite good (http://caniuse.com/#search=rem)
and version 3.0 will drop IE8 support.