Cover map

Replaces the SVG map with a MapBox-based slippy map:

![screen shot 2013-09-03 at 3 44 17 pm](https://f.cloud.github.com/assets/261584/1075979/27b51864-14da-11e3-83d9-7769a954a1e7.png)

Two issues remaining:
- The tiles here are using my own private free MapBox account with a 5k tile per day limit. MapBox is relatively expensive, but rendering basemap tiles is probably out of our scope...
- The GeoJSON returned by the API is oversimplified for small geometries. I have a [story in Pivotal](https://www.pivotaltracker.com/story/show/56167316) for this.