handle access_token expiration/invalidation

[rafaeltuelho]

I believe we should verify if the current access_token remains valid before do an API Call. After token expiration (defaults to 60min) time I received an HTTP 401 error because the current token is invalid. I was able to workaround this by invoking GAuth.checkAuth() before calling my APIs. But I believe the right way would be usin refresh_token. Or maybe using the token_info endpoint to validate the current token before each API call.

https://www.googleapis.com/oauth2/v1/tokeninfo?access_token={accessToken}

What do you think? Maybe I just didn't get how this is being done here...