Implement GeoJSON

* Create GeoJSON in Incident model
* Get GeoJSON in homes controller, remove logic from model
* Change JavaScript to accept GeoJSON to create markers
So what I mean is that i'd rather be able to do something like:

```
class GeojsonBuilder
  def initialize(incidents)
    @geojson = []
    @incidents = incidents
  end

  def all_markers
   @incidents.each do |incident|
      # ...
   end
end
```

Then your dependency on `Incident.all` is removed.