Support openid.realm parameter

Google has written migration guide for switching from OpenID 2.0 to OAuth 2.0. Everybody needs to migrate as Google's OpenID 2.0 support ends in 2015. Guide is available at https://developers.google.com/accounts/docs/OpenID. In step 1 on that guide, openid.real parameter is added to the initial authentication request.

This PR adds support for openid.realm option.