Improve Aura instantiation

We now always turn the Aura attribute into a class, to allow for
virtual aura cards.
Copy the buffs list when destroying everything

buff.destroy() removes items from self.buffs... oops.
Remove aura enchantments from minion that should no longer be affected

This ensures minions dying or going back to the hand lose their aura
at the appropriate moment and the aura does not try to remove it
long after they are already dead.