small UI fixes and Sass usefulness

Hi, some small CSS improvements mostly here.

* Added variables for all the Code of America official colors (thanks Harlan for the [style guide](http://style.codeforamerica.org/)!)
* Wrote a mixin to easily create a large colored block of content with a specified background color and text color
* Incorporated that mixin into the masthead and headline classes to make standardization across pages a bit easier.. 
* Fixed the UI issues pointed out in #27 but overall we still need to make the site more responsive ;)

As a bonus, comes bundled with merged Liam's pull request #30 (which I liked, but I simplified the markup for the featured event a bit)