REST API should support reverse-geocoding

[slinkp]

Currently, there's no way to ask openblock's API what address it would use to describe a point.

Proposed API:

GET /api/dev1/geocode/reverse/?lat=100.0&lon=-100.0

Proposed response: A GeoJSON feature with the address in the 'location_name' property, for symmetry with other GeoJSON features in the API.

    { "type": "Feature",
      "geometry": {"type": "Point", "coordinates": [-100.0, 100.0]},
      "properties": {
        "location_name": "100-147 Main St",
        }
    }

This is a prerequisite for #284

[slinkp]

Ticket imported from Trac:
http://developer.openblockproject.org/ticket/285
Reported by: slinkp