Fixed #13315, #13430 -- Recreated `django.contrib.gis.db.backend` module with...

Fixed #13315, #13430 -- Recreated `django.contrib.gis.db.backend` module with `SpatialBackend` alias and added `Adaptor` alias for backwards-compatibility purposes; added GeoDjango 1.2 backwards-incompatibility documentation and release notes; added a section in the docs about how MySQL is a crippled spatial database; updated versions in install docs.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@13097 bcc190cf-cafb-0310-a4f2-bffc1f526a37