Expose Authenticator callbacks

[mjohnson9]

As the title says, it'd be nice to have the ServerAuthenticator and ServerUpdatingAuthenticator callbacks exposed. This would allow one to implement an authenticator as a Mumo module rather than having to rewrite most of the boilerplate handling Ice disconnects and such.

Unfortunately, at present, it doesn't seem to be possible at all to implement an authenticator within Mumo because the Ice adapter is never exposed.