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 bug of django or some unexpected backward incompatibility. For avoit 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