Fixed #12591 -- Renamed `add_postgis_srs` to `add_srs_entry` (keeping...

Fixed #12591 -- Renamed `add_postgis_srs` to `add_srs_entry` (keeping backwards-compatible alias), added `database` keyword for multi-db, removed deprecated `SpatialBackend` references; no longer use `get_or_create` and look for existing entry based on SRID value. 


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12228 bcc190cf-cafb-0310-a4f2-bffc1f526a37