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`
The named example is rather unlikely to happen, isn't it? I filed #61 to keep track of what we should do when the user clears their cache.