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 database loss example is unchanged from earlier versions of the spec. I'm not sure how a user agent would even be notified if the registration data was lost... how would the server know who to notify without the identifying data? I'd like to discuss that with the people that came up with this example.