Addresses broken PostGIS support.

When using PostGIS pooling backend, PostGIS opens temporary connections to the DB to query PostGIS versions - this happens during the `__init__()` of `DatabaseWrapperMixin16` when calling its `super`. Due to the class/mixin inheritance, it's using the new `get_new_connection` and `close` methods from `DatabaseWrapperMixin16`, but `self.pool` isn't initialized yet.

This commit addresses this scenario - it is largely UNTESTED however. I know that `manage.py runserver` actually runs now with PostGIS backend.  Also - `SOUTH_DATABASE_ADAPTERS = {'default':'south.db.postgresql_psycopg2'}` is needed in your settings.py when using the PostGIS backend.