Update OSM.pm

Hi,

I found your module while searching for a perl GeoCoder module to use with OSM. As I want to use it for reverse geocoding as well I found that it does not contain such a function, yet osm and mapquest do support it. As Geo::Coder::Google::V3 has a function 'reverse_geocode' it seemed like a good idea to add it to your module as well. So I did, changing some other bits of code as well to make it fit in.

Hope the change is to your satisfaction and you are able to push it to CPAN.

Kind regards,
Frank