SW Sample: Offline Analytics

@jakearchibald @ebidel & co.:

This builds on an existing sample that demonstrates offline functionality, and addresses the question of how to get a system like Google Analytics that relies on sending data to a remote server to play nicely with offline web pages.

The code queues failed Google Analytics pings in `IndexedDB` and attempts to resend them each time the service worker starts up, with the `dt=` parameter set to reflect the original time of the page view event.
Is a 500 something one should retry? If that URL continues to 500, won't this continue to be retried every time the SW starts?