title: Firebase Authentication Service - lookup uid from email without login
body: <p>I am building an app using Firebase, Firebase's Authentication Service (simple email/password login), and storing user information in a manner pretty similar to what is described <a href="https://www.firebase.com/docs/web/guide/user-auth.html#section-storing" rel="nofollow">here</a>.</p>
  
  <p>Part of my functionality will need to allow looking up another user based on an email address.  </p>
  
  <p>Is there a straightforward way to use an email to lookup the Firebase defined uid (i.e. 'simpleLogin:9') without having to iterate over all users, or being able to actually log them in?</p>