Fixed authentication using VK

Fixed authentication using VK as reported in https://github.com/gae-init/gae-init-auth/issues/34 and in the process updated the end-point used to `/method/users.get` (see http://vk.com/dev/users.get) instead of the previously used `/method/getUserInfoEx` (which is now listed under *"Outdated Methods"* at https://vk.com/pages?oid=-17680044&p=API_Method_Description).