Implement GeoJSON

* Create GeoJSON in Incident model
* Get GeoJSON in homes controller, remove logic from model
* Change JavaScript to accept GeoJSON to create markers
Given the method name is `build_geojson` I would expect a JSON object to come from this rather than a hash. In fact I would suggest following a pattern that Rails has which objects provide methods such as `as_json` and `to_json`. Where `as_json` returns the Ruby hash, and `to_json` returns the actual JSON.