title: Can I Firebase createUser with an arbitrary ID instead of an email address
body: <p>I'm using Firebase for an Atlassian Connect AddOn. I want to use Firebase users to secure the data.</p>
  
  <p>The users will be identified by a clientKey provided by Atlassian (probably after I fudge it a bit) - NOT BY EMAIL.</p>
  
  <p>Ideally, I don't want to have to do my own authentication here, as the Firebase.createUser method would suffice entirely if I could provide something other than an email to it, but I can't find anything like that in the documentation.</p>
  
  <p>Is there a way I can create Firebase users WITHOUT AN EMAIL (just an ID and password), without going to all the way into oAuth and all that jargon to create my own custom authentication?</p>