Add functions to parse response from Google OAuth2

Make the Mixin easier to get data from encoded response
parse_response is a generic name for a fairly specific method.  Maybe something like parse_user_info?  

This doesn't look like it needs to be an instance method; would it make sense for this to be somewhere else?  Maybe a more generic function that could be called as jwt_decode(response['id_token'])?

Is this format google-specific or might it be useful for other oauth2 clients too?