Add doc about how to customize navbar
Branch: bug/296-navbar
Allow for customizing nav bar more
Branch: bug/296-navbar
Refs #296: more style changes to navbar

- make the words in the responsive menu white on blue instead of gray on blue
- change the responsive menu button from solid black to a gradient to the same
  shade of blue used elsewhere.

Branch: bug/296-navbar
Merge pull request #296 from rapidsms/bug/296-navbar

Customize and clean up navbar