Get rid of PgModel, replacing it with simple signals

Get rid of PgModel, replacing it with simple signals

We were already using signals for everything except delete, and even
in our old version of django the delete signal exists (it didn't exist
when this code was first written).

Django doesn't really like models to be OOP like this, so keeping PgModel
would cause issues with upcoming changes in django 1.8. Using simple functions
is easier, and the actual functionality is replicated straight off.