PIP: GeoJSON for Features

This is work in progress, up for discussion.

Trying out a couple of things for GeoJSON serializers. The thought is since we're going to be really promoting using spray with GeoTrellis we should have should consider integration with their Marshallers/Unmarshallers.

Concerns are:
1. Integration with spray
2. What will user have to do to handle non-standard features like PointFeature[BlackBox]
3. How is CRS going to be pulled in for during serialization.
4. ??? Anything else ?