#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.
I think using `sys.modules` in that situation is slightly overkill. You could just have a dict of factories.