Patched Geoalchemy for PostGIS 2

To get CKAN 2.3 master (Dec 2014) to work with ckanext-spatial on a PostGIS 2.1.2 db (Ubuntu 14.04 FWIW), this patch uses a GelAlchemy version by @chokoswitch as I mentioned in #5 
@amercader is of course right to suggest upgrading CKAN's SQLAlchemy - in the mean time, here's a patch that works at least for me.