Add page callbacks to both authenticate with and log out of Google

Issue Summary

If would br great if this module had callbacks such as "google_auth/login" and "google_auth/logout" that would authenticate with / log out of Google respectively. The use case could be that if the user wasn't logged into to Google, they could be presented with al link prompting them to do so. The link would point to "google_auth/login" and and have the page the user came from in the query string. For the purpose of this discussion let's call it "current_page". The page callback would then redirect to Google so the user could authenticate, which would then throw the user back to "google_auth/login" which would redirect to the page in the "current_page" variable.

This would provide a consistent method for all modules extending google_auth to use instead of having to explicitly code the authorization in the module.

Thanks for a great module,
Chris