Clean up events and define relationship to service workers

This relates to issues #33 and #18.

* Define event handlers `onpush` and `onpushregistrationlost` on `ServiceWorkerGlobalScope`
* Define `push` event using interface `PushEvent`
* Define the simple event `pushregistrationlost`
This needs a reference to the Service Worker specification. A concept of "starting a service worker" is too large to leave without a link.