Remove Registration.createToken private modifier

I have a use case of your plugin where I had to copy past the controllers.Registration.createToken method because of it's private modifier, it would be nice to have it public :).

[This](https://github.com/SlickChair/SlickChair/blob/master/app/controllers/LoginWrapper.scala#L76) is the code I'm referring to. What I'm doing here is to use a single form to handle login, creation of email accounts and "forgot password". I only care about having verified emails of the users, and in this context I don't need to make the traditional sign up/sign separation.