xstream.aliasField( "remote-port", PortForwarding.class, "remotePort");
xstream.aliasField( "local-port", PortForwarding.class, "localPort");
xstream.aliasField( "host-string", HostKey.class, "hostString");
Writer strWriter = new StringWriter();
PrettyPrintWriter prettyPrintWriter = new PrettyPrintWriter( strWriter );
xstream.marshal(configuration, prettyPrintWriter );
System.out.println( strWriter.toString() );
}