Web authentication requires access to Google+ API

[abstrctn]

The after callback in the authentication module makes a call to the Google+ API to retrieve the authenticating user's email address for identification.

Current instructions do not include Google+ as a required API. Not enabling it results in an error that is not surfaced, and if web authentication is required using the DRIVESHAFT_SETTINGS_AUTH_REQUIRED setting, would result in an authentication redirect loop.

TODOs:

• Add use of Google+ API to the docs
• Surface error as a flash message when it occurs
• Possibly include a page where flash messages can be shown for apps with AUTH_REQUIRED turned on

I'm guessing this is the cause of #15 and #12.