Added OpenStreetMap Nominatim gazetteer option

This is a bit of a mess of commits and merges (to reconcile my fork with the master) but the basic point is that I've added the option to search the OpenStreetMap online nominatim gazetteer. The config file for this gazetteer could be modified easily to point at a local version, or could include the email address of the requester as per http://wiki.openstreetmap.org/wiki/Nominatim