Use custom SQL instead of models to fix maps download bug

Fixes https://bugs.launchpad.net/oq-platform/+bug/1395784

With this PR we use custom SQL to be sure that the ```the_geom``` column is in the 2nd position. I also cleaned ```models.py``` to be more pep8 compliant. 

![screenshot from 2014-11-27 12 03 33](https://cloud.githubusercontent.com/assets/1818657/5215998/7eb40980-762d-11e4-8924-1d1cf7830a8d.png)