title: Firebase and React - check user auth status
body: <p>I'm working on app based react+mobx and as backend use the firebase. 
  Currently on the app start i need to check if user is logged in. 
  I'm doing it with firebase.auth().onAuthStateChange listener, but because it's async function - the first response always null. 
  So my current solution is display some loading message and put setInterval function with status checking in componentWillMount lifehook. 
  The question is - is there any more elegant solution to this issue?</p>