Include full-width color mixin

This lets the user define a full-width background color [within a max-width container](http://css-tricks.com/full-browser-width-bars/). I've also created and documented a [nifty mixin for full-width imagery](http://www.commercekitchen.com/2013/10/full-width-backgrounds-scss/). I can send another pull request for this if you're interested.

Finally, I didn't get the test environment spun up yet, but this is [functional on CodePen](http://codepen.io/mragray/pen/BKlIp).