Fix how SQLAuthLogic and SQLRestAuth fetch their user model class.

I have not tested this because I'm about to get on a bus, but I believe this patch will fix issue #225 or at least approximate the correct patch.  The class method `user_model` has been replaced with `user_models`, which is a hash that keys user model classes by an authenticator's `auth_index`.  The base SQL authenticator has a method called `user_model` that will fetch the correct user model class.

Could someone try it out?