Add in Google OAuth2 support.
Fix up documentation for the class and the Sphinx reference.
Rename the object param in GoogleOAuth2Mixin from overwrites to extra_params.
Stop making an absolute URL for GoogleOAuth2Mixin
Clean up GoogleOAuth2Mixin, fold scope and response_type into the base OAuth2Mixin
Merge remote-tracking branch 'sclm/GoogleOAuth2' into merge

Conflicts:
	docs/releases/next.rst
Rearrange newly-added optional arguments for backwards compatibility.