use psycopg2 connection as register_hstore_handler argument

this is a minor issue and does not help with #11, but the `connection` handed over to `register_hstore_handler` is a Django wrapper and so is the cursor. `register_hstore` expects a psycopg2 connection or cursor. It does not matter as long as `globally=True`, but with `False` psycopg2 complains about an unexpected argument type.