Model Spot now enforces compliance with GeoJSON feature spec.
Model Spot now has a static function fromLatLng(latLng) to create
spots from just that.
MapView#setMarkerAtCurPos now adds a Feature to the GeoJSON layer.
This means that the new Spot will directly have the openPopup
callback because it is applied to each feature of that layer.
Furthermore MapView is now responsible for creating a new Spot and
FormView no longer has a clue about the Spot model.