Respect Google+ API rate limits

[rjoonas]

We have an application that utilizes this module and deals with quite a few requests. At peak traffic it will sometimes exhaust the Google+ API quota for the application. When this happens, express-gauth's error handler will log the following (at index.js:130):

GAuth error { GooglePlusAPIError: User Rate Limit Exceeded
message: 'User Rate Limit Exceeded',
name: 'GooglePlusAPIError',
code: 403 }

It'd be useful if we could provide express-gauth with our Google+ API quotas and it would throttle requests. Retrying with an exponential backoff delay might help as well?

Default quotas I'm currently seeing in Google Developer console for Google+ API:

• 20 000 000 sign-in queries per day
• 500 sign-in queries per 100 seconds per user
• 25 000 sign-in queries per 100 seconds

Of course the developers can always request a more lofty quota from Google. :)