[WIP] Add source geometry

 * [ ] Make it possible to add a GeoJSON feature to any source and have it rendered.
 * [x] Switch to a whole-world render up top instead of just US:
    
    ![world](https://cloud.githubusercontent.com/assets/58730/5426263/8f7f0ef6-82fb-11e4-9dfe-80e45969e9e0.png)