Added models validator for REQUIRED_FIELDS not being a list.

[romanalexander]

I spent some time debugging django-admin.py createsuperuser raising optparse.OptionConflictError. I forgot to make REQUIRED_FIELDS custom user model a list. This validation error should help others avoid my mistake.

[charettes]

You should open a ticket in trac and link it to this PR. We mainly use PR to review patches.

Concerning the patch, shouldn't tuple be also allowed or even preferred in this case?

[romanalexander]

Sorry, I had read in the docs that a ticket wasn't necessary if its a small update, and I thought this fell in that case.

I'm sure tuple would also be acceptable, but I was just followed the documentation.

Edit: Ticket submitted to trac here.