Google oauth 2

Support OAuth2 flow and Directory API.

Google has deprecated OAuth flow for any new application, so the migration to OAuth2 is just a matter of time. Moreover, Google has deprecated the Provisioning API in favor of the Admin SDK.

This PR introduces the usage of OAuth2 flow and Admin SDK APIs. This is a breaking change given the Gaah API will not be compatible with 0.3.x version - that's now on its own branch.

Clients need to upgrade to the new version (that's why we are skipping 0.4) in order to support it. The README explains how to handle OAuth2 auth flow in an app.
The data format between GAAH and App hasn't changed, helping the migration to the new API.