Social Sign In On iOS

References
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1. Swift 3 + Firebase Authentication - Google Sign In
https://www.youtube.com/watch?v=83EXULT480Q

2. Swift 3: Firebase Social Login - Google Sign In (Ep 4)
https://www.youtube.com/watch?v=rDKtMz_PcLQ

3. Swift 3: Firebase Social Login - Facebook Authentication and Cocoapods
https://www.youtube.com/watch?v=iSszeW1aH6I

4. Swift 3: Firebase Social Login - Facebook Email and Custom Login Button (Ep 2)
https://www.youtube.com/watch?v=Tl_uCGjCZd8

5. Sign in series #2: Adding Google Sign-in to your iOS App (Route 85)
https://www.youtube.com/watch?v=QmnI5c85sf0

6.
http://stackoverflow.com/questions/39550525/firinstanceid-warning-stop-will-reset-deviceid-from-memory-xcode-console-lo/39560078#39560078