Currently all users can set one OpenID identity that allows access to the account.

The identity url's for the server is owncloud/apps/user_openid/user.php/$username

Move documented functions for reference into OC_USER_EXAMPLE class.

keeping tracked of the logged in user is no longer done by the active backend but by oc_user directly instead

multiply backends can be active at the same time, allowing alternative authentication procedures like openid or tokens to be used next to the regular user system