Create a predicate to match passwords

This addition is in response to the rejection of a previous pull request because it changed an existing method:
https://github.com/rails/rails/pull/15153

My hope is that since it is additive there won't be as much hesitation to include it since it won't break existing code.

This does add a bit of surface area to the model using has_secure_password. Hopefully this addition seems worth it for the added consistency of the API.