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`
s/push server/push service/. Push Service is defined as the front-end an application server talks to, whereas this would in many cases be the independent backend.