Google+ Sign-in button

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• Check for stored credentials on first login
• Check for stored GCM credentials as a bonus
  1. Go Google+ API client: https://github.com/google/google-api-go-client
  2. "If you do not require offline access, you can retrieve the access token and send it to your server over a secure connection. You can obtain the access token directly using GoogleAuthUtil.getToken() by specifying the scopes without your server's OAuth 2.0 client ID" https://developers.google.com/+/mobile/android/sign-in (bottom paragraph)

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Topic moved here: https://developers.google.com/identity/protocols/CrossClientAuth and https://developers.google.com/android/guides/http-auth

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Also useful:

• https://github.com/googleplus/gplus-quickstart-go
• https://developers.google.com/oauthplayground/

And:

• https://developers.google.com/identity/sign-in/android/
• https://github.com/googlesamples/google-services