Update models.py

This will give people an option to 'logout'. so making a handler to process the delete() can call this method to remove the token.
Do you envision this method being called when a `User` instance is available? My guess is that a logout endpoint would unfold like so:
 * Verify the provided access token and retrieve the current user
 * Delete the access token

If that flow is correct, would it be more appropriate to bind this method to the instance? e.g.
```python
def delete_bearer_token(self, token):
   self.token_model.get_key(self.get_id(), 'bearer', token).delete()
```

Feel free to disagree. And thanks again for the PR!