https://github.com/mkdynamic/omniauth-facebook
https://github.com/zquestz/omniauth-google-oauth2

Create Google+ application:
  https://developers.google.com/+/web/signin/javascript-flow

Create Facebook application:
  https://developers.facebook.com/docs/facebook-login/v2.1
  https://developers.facebook.com/docs/reference/dialogs/oauth