Pass-through caching demo.

@kenjibaheux @jakearchibald @slightlyoff & co.:

Here's my initial attempt at a pass-through caching example, closing #31.

You can see a live demo of this code at https://rawgit.com/jeffposnick/samples/sw-pass-through-caching/service-worker/pass-through-caching/index.html
Thanks, that's very true.

What's the limitation on Cache API storage that we're planning on enforcing with Chrome 40? And is that limit enforced based on cache name, service worker registration, or something else?

I'd like to include some actual numbers based on the Chrome implementation, and will add the caveat that other browsers or future versions of Chrome might enforce different limits.

I'll also add something about how naive pass-through caching is appropriate for web apps that make a limited set of HTTP requests, but web apps that might end up making arbitrary HTTP requests (like, say, an RSS client or news client) shouldn't blindly pass-through cache everything and need to periodically prune their caches if they do cache aggressively.