validator class from oauth2_settings

Hello,
We would like to be able to use django-oauth-toolkit for our project, but unfortunately we use very different kinds of models and the validator doesn't fit our needs.
As suggests this pull request, we would like to be able to specify as a setting the class used for the validator (along with the models used by this validator).
We hope you will find it convenient.
Anyhow, thanks for the good work.
Have a good day,
M