Add mapbox attribution

Based on: http://bl.ocks.org/lxbarth/9490377

(But made the logo darker to standout more on our white map)

![screen shot 2014-06-25 at 4 27 00 pm](https://cloud.githubusercontent.com/assets/1435836/3386499/121bd398-fc75-11e3-8b4c-b9bc6a66b5a4.png)