Adds a third option for strict to raise a Validation Error

I supplied this third option so that Django Rest Framework could return a 400 w/ an appropriate error message.

If this is to your liking, Alex, I'd be glad to add some unit tests before you merge.

Note the bizarre constants True, False, "RAISE" -- I did this to support backward compatibility.  If you recommend a cleaner way to manage the strict options, let me know. Someone here recommended another boolean option, but then there'd be 4 possible values of the two booleans, only three of which make sense.  I think this is more robust.