Add default message handler to WorkerPar

[lars-t-hansen]

At the moment this idiom is used everywhere:

var Par = new WorkerPar();
onmessage =
    function (ev) {
        if (!Par.dispatch(ev.data))
            postMessage("Unknown: " + ev.data);
    };

It would be reasonable to support this idiom to allow simply this:

onmessage = (new WorkerPar()).defaultMessageHandler;

Even if theWorkerPar instance is needed (eg for Par.self) and has to be captured this is an improvement.