Added field() method for geofilters; added class to sort by GeoDistance

Seems that the documentation in elasticsearch regarding pin.location is just a default they used - which doesn't mean that other developers will actually follow that example.
I've added the possibility to change the name of the field containing the geocoordinates with a fallback to pin.location.

There's also a new sorting class to sort by GeoDistance.

I hope this is useful.