Additional bootstraping on hstore.py module.

I have found some strange behavior on get django-hstore working under gunicorn and uwsgi without this patch.

In some strange circumstances, django does not loads django_htore/models.py (using django 1.6.6), it can be a bug of django or some unexpected backward incompatibility. For avoid it, is much better duplicate the hstore bootstrapping code in two major entry points on django hstore for django < 1.7.

And the second commit, is removing now obsolete code. Because the last required psycopg2 version has buitin (and possibly much better) version: http://initd.org/psycopg/docs/extras.html#hstore-data-type