Use SFSafariViewController

As an App Developer, I want improved OAuth2 iOS lib to avoid MITM attacks.

Improve OAuth2 iOS lib using iOS9 SFSafariViewController which mitigates MITM risk by providing a read-only webview.

https://developer.apple.com/library/ios/documentation/SafariServices/Reference/SFSafariViewController_Ref/
https://developers.googleblog.com/2016/08/modernizing-oauth-interactions-in-native-apps.html