Feature geometry should be constrained to Geometry type

[jasonCodesAway]

Currently Feature.java has a private GeoJsonObject geometry;. This type should instead be private Geometry geometry; per section 3.2 of the geojson spec:

A Feature object has a member with the name "geometry". The value of the geometry member SHALL be either a Geometry object as defined above or, in the case that the Feature is unlocated, a JSON null value.