SMS-based authentication provider

[gmethvin]

It would be good to have a way to authenticate users via SMS as well. On mobile apps it's becoming popular to not require a username/password and do the signup/login process by sending an SMS. It seems like the implementation would be similar to the username password login except you would provide a userid/username and a single-use token, and there would be some mechanism by which you can request a new token.

I've been using the passwordless module to do this in node, but there doesn't seem to be a nice equivalent for Play.