Twitter register removal

This feature allow users to associate other auth providers with their profile.
It also removes the possibility of a user to create an account from a twitter authentication. Twitter still been a ok way to authenticate, but the profile should exist (be registered by other provider), the users will have to associate the twitter account, and only then they can use twitter as a auth provider.

This is a solution for the problema described on #71. But more elegant than the one suggested there.