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.
Why not just rename the variable in the for { } above to `writer`?