Fixed #3460 -- Added an ability to enable true autocommit for psycopg2 backend.

Ensure to read the documentation before blindly enabling this: requires some
code audits first, but might well be worth it for busy sites.

Thanks to nicferrier, iamseb and Richard Davies for help with this patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10029 bcc190cf-cafb-0310-a4f2-bffc1f526a37