stop using mptt.register and deprecated/unused AlreadyRegistered exception in sample model

I'm about to release mptt 0.6. I almost removed mptt.AlreadyRegistered, but luckily I ack'd the django-cms codebase and it's still in the sample models file.

AlreadyRegistered hasn't been thrown since mptt 0.3, so not much point catching it. I took the liberty of switching to MPTTModel instead of mptt.register() too, since it's a bit prettier.