Implement GeoJSON

* Create GeoJSON in Incident model
* Get GeoJSON in homes controller, remove logic from model
* Change JavaScript to accept GeoJSON to create markers
How about another method on `GeojsonBuilder` called `to_json`. Something like:

```ruby
class GeojsonBuilder

    def to_json
        @incidents.map do |incident|
          incident_as_json(incident)
        end
    end

    private

    def incident_as_json(incident)
      {
         # ...build hash here
      }
    end
end
```