Add error checking for Mapquest lookup (tests included)

Mapquest returns some status information that we can examine to see if the request was successful.  It's the same status for both the licensed and open apis.

http://www.mapquestapi.com/geocoding/status_codes.html
http://open.mapquestapi.com/geocoding/status_codes.html

Patch adds checks for the status and raises (or warns) with the relevant error (modeled after the Google lookup).