#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.
After a second review I realized it is not necessary to remove. MongoDB only supports some elements of GeoJSON. I mentioned in a previous comment.

> I don't find useful to support Features Objects or Features Collections. As @kynan pointed before CRS Objects are not required as MongoDB uses WGS84.