Support reverse geocoding?

We'd like to be able to make authenticated Esri reverse geocoding requests in OpenTreeMap.

Authentication with the Esri geocoding APIs is somewhat complicated, and since we [plan to support it for geocoding](https://github.com/azavea/python-omgeo/issues/35), it would be nice to lean on that infrastructure for reverse geocoding as well.

Do we want to support reverse geocoding _at all_ in this library? Would a new library dedicated to reverse geocoding be more appropriate?

Assuming we do add support for reverse geocoding, it raises a lot of API questions:
- Are there any desired pre/post processors for reverse geocoding?
- Would reverse geocoding use the same service settings as geocoding? The API keys will be the same for a given service, but desired pre/post processors would likely be different for reverse geocoding than they are for normal geocoding.
- Do we add a separate `ReverseGeocoder` class, as a companion to `Geocoder`? Or do we add a `reverse_geocode` method to `Geocoder`?
- What services would we like to to support reverse geocoding in besides EsriWGS?