Enable contrib.gis to allow OpenLayers forms on the Django Admin site

Needed for editing things like Events which contain geospatial data.
Without this in place and without a custom form for editing geospatial,
the Django Admin page will *crash* on models that have Geospatial fields
Merge pull request #119 from maurizi/enable-contrib-gis

Enable contrib.gis to allow OpenLayers forms on the Django Admin site