Tie OAuth user to Parse user

[JonBoley]

When a person uses external auth service (e.g., Google/Untappd/Twitter), that needs to map to a specific user in Parse

[JonBoley]

from http://blog.parse.com/2013/12/03/bring-your-own-login/

Parse.User.become("session-token-here").then(function (user) {
  // The current user is now set to user.
}, function (error) {
  // The token could not be validated.
});

Also, check out this tutorial: https://parse.com/tutorials/adding-third-party-authentication-to-your-web-app