Logout always deletes authtoken, regardless of token settings

[charles-hussey]

Even if the TOKEN_MODEL setting is set to None, the user's Auth Token will be deleted whenever they log out.
I had wanted to use this package as an easy way of adding JSON versions of the standard auth endpoints (while maybe adding social auth in the future). However, I currently use auth tokens as a secondary means of authentication so users can script things more easily. It obviously won't work for me if the auth token gets deleted every time, nor can I see the rationale in doing so if the TOKEN_MODEL is set to None.