Adding method required by Devise Token Auth.

[CodeTectonics]

Thank you very much for creating and maintaining this gem. I have just started using it for Active Directory integration and it is working very well.

My stack also includes Devise Token Auth and I needed to add "valid_password?" to the model in order to make the gems work together.

Using these gems together seems like it would be quite a common practice, so I thought I would make a pull request to add this method.