Allow model object/instance pk to pass to uuslug

It is sometimes useful to allow model class/instance pk to be passed to the uuslug method rather than just an instance. I came across this issue during a South datamigration where the model instances don't conform to the "objects" check currently in uuslug, so passing the orm frozen model and pk was the solution to get a workable instance. Alternatively, could use a different check to differentiate between models and instances, but wasn't sure of another good way to do that. So I created this instead.