Update to Google Oauth 2.0 login 

per the instructions at:
  https://developers.google.com/+/api/auth-migration#userinfo

This has not been extensively tested, but has been lightly tested in our application.
You already have a unique identifier by calling the ```getId()``` method of the profile, it will return the same value: https://github.com/leleuj/pac4j/blob/master/pac4j-oauth/src/main/java/org/pac4j/oauth/client/Google2Client.java#L106. It's just strange to get a number for a username, it's more a technical identifier.