Re: [push-api] Rename (un)register to (un)subscribe (#98)

The other SW APIs are less related to messaging though, so perhaps it's ok to be named differently? i.e. perhaps the following is actually fine?:

```js
navigator.serviceWorker.register(...);
navigator.serviceWorker.ready.then(registration -> {
    registration.pushManager.subscribe().then(...);
});
```

I suppose it does conflict a little with the `serviceWorkerRegistration.syncManager.register` API currently suggested for Background Sync in slightlyoff/BackgroundSync/issues/20, though there's still time to change that.

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