Clean up the JavaScript example

This brings the example much closer to current discussions. This does not yet take into account the idea of moving pushRegistrationManager to ServiceWorkerRegistration as I think Peter had some questions about that.

This fixes issue #17.
Perhaps clarify the comment a little bit to (a) indicate where the application server needs to send the messages to, and (b) prevent making good practices sound like requirements?

"The push registration details needed by the application server to push messages to the push service are now available, and can be send to the application service using, for example, an XMLHttpRequest."