#299: Geopoint field type

As discussed in issue #299 I added functionality to define in schema geojson types. Validations added.
http://geojson.org/geojson-spec.html

TBH I was thinking of adding indexing as well, using geohashing, but I prefer to pull by parts.
Ideally this would also validate that the position is in the valid range for the chosen Coordinate Reference System (CRS), but I don't have a good suggestion for how to do it in a general way. You could assume the position is longitude, latitude since MongoDB defaults to WGS84.