Support export of geo datasets as GeoJSON

- As it turns out, Geotools support for serializing FeatureCollections to GeoJSON is quite complex and unwieldy. I ended up just using Rojoma JSON as the code to serialize to GeoJSON using the Geotools API was about 5x as long, much of it boilerplate.
- The reason there's so many new files is that I split out all the exporters to their own files. I haven't actually changed anything in JsonExporter, CJsonExporter or CsvExporter.
I think embedding classes inside the yield block, and large yield blocks in general, are pretty yucky.   How about getting rid of the class, moving the functions outside to the Object (or at least inside the def export)?  The "go" code can just be what is inside the yield.