Raw confirmation token docs and test

Guys,

today when I send a confirmation with `User.find(1).send_confirmation_instructions` the user object has the `raw_confirmation_token` attribute filled. I cant find a documentation of this, I added to the RDoc, I really dont know if this is the best syntax to add this, I need to define a acessor, not change the actual behavior.

I added a test to this, since it is a public interface of `Confirmable`.
I'm not sure that's supposed to be accessible externally, but in any case, it might probably be a reader only here.