Added savepoint support to the transaction code.

This is a no-op for most databases. Only necessary on PostgreSQL so that we can
do things which will possibly intentionally raise an IntegrityError and not
have to rollback the entire transaction. Not supported for PostgreSQL versions
prior to 8.0, so should be used sparingly in internal Django code.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8314 bcc190cf-cafb-0310-a4f2-bffc1f526a37