fix for google users who haven't signed up to google plus

Hi,

I found that the google authentication page allows any google account to login but the 'plus' webservice fails for users who haven't signed up to google plus. So these users get errors with flask-social when they try to login with google.

The same information that is fetched from the 'plus' webservice can be retrieved from the 'oauth2' webservice instead. I've submitted a patch to do that.

You could argue that in the long run all google users will probably be forced to sign up for google plus account, but I'd argue right now that isn't the case, and the anomaly of being able to select an account and have it throw an error is a bad user experience.

Cheers,
Dan.