Fixed migration custom User compatibility.

Hi,

It appears as if custom User model support was broken by #387.  I restored the fixes.  The only migration that does anything with the User Model is 0001_initial, I just made it point to the user_orm_label again.  While the other migrations do have 'auth.User', in their frozen models, they don't actually touch or query the user model, so they don't fail when using a custom User model.

Thanks,