Renamed `reverse` to `reverse_geom` because of clash with existing `QuerySet`...

Renamed `reverse` to `reverse_geom` because of clash with existing `QuerySet` method; added `GeoQuerySet.geohash` support for producing GeoHash reprsentations.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12350 bcc190cf-cafb-0310-a4f2-bffc1f526a37