Added experimental support for a POSTGIS_VERSION setting for GeoDjango. If...

Added experimental support for a POSTGIS_VERSION setting for GeoDjango. If set, it will tell GeoDjango not to do the postgis_lib_version() query every time the server starts up (assuming a PostGIS backend). This is intentionally undocumented because Justin mentioned he might be refactoring the backends for Django 1.2 such that this setting would be unnecessary. Until then, people can use this hook.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10549 bcc190cf-cafb-0310-a4f2-bffc1f526a37