Made sipsimple.core.Invitation properly subclassable

darcs-hash:20090416161907-58103-19fdfceb6c60b4ccdce07a02902f3b52e8547822
Made sipsimple.core.AudioTransport properly subclassable

darcs-hash:20090416162540-58103-100ad8e92b08b9db140d773fc642d8e33ef970ba