Implement GeoJSON

* Create GeoJSON in Incident model
* Get GeoJSON in homes controller, remove logic from model
* Change JavaScript to accept GeoJSON to create markers
It feels like a code smell that the controller needs to know to call `all_markers` on the GeojsonBuilder before being able to call geojson.

Also, I feel like your doing extra work here in the controller calling `to_json` on the content returned by `geojson`.  Perhaps just being able to call `marker_builder.to_json`