Ideas for Push API recipes

- [ ] Registration (Google Cloud Console, manifest file, need to register from document first, sending data to their server)
- [ ] Sending a simple message (Perhaps a very simple back-end example for sending something to GCM, similar to what Matt is using)
- [ ] Receiving a message (The "onpush" event, why no payload, using fetch())
- [ ] Displaying a notification (Why? Good practices for engaging users (icon + title + body), store data in IDB, maybe "tag" hack?)
- [ ] Handling a notificationclick even (focusing or opening windows)

These are following a sync with Peter a week or so ago. We've since opted to ship a single larger sample in #79, but these could still be useful to explore when time allows.