title: Devise sign up either by email or by mobile number
body: <p>I can register a user getting both email address and mobile number in my application. But what I really want is to register user using either email or mobile as a primary authentication key. So that if a user provides email address, it must be saved in email field in database and if user provides mobile number, it must be saved in mobile field in database. </p>
  
  <p>And also I would like to overwrite the confirmation method of mobile user and would like to send an message with activation key and inter the key in the application to activate registration. I think its not so hard, but I didn't get where to start. Please suggest me the favourable way to accomplish this task.</p>