#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.
@kynan Thanks for the review!!

My implementation is focused in GeoJSON objects accepted by [MongoDB](http://docs.mongodb.org/manual/applications/geospatial-indexes/):

- [x] Geometry Objects 
       - [x] Point
       - [x] LineString
       - [x] Polygon
       - [x] MultiPoint
       - [x] MultiLineString
       - [x] MultiPolygon
       - [x] GeometryCollection
- [ ] Feature Objects
- [ ] Feature Collections

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] (http://docs.mongodb.org/manual/reference/glossary/#term-wgs84).