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
> naive pass-through caching is appropriate for web apps that make a limited set of HTTP requests

Although apps that make a limited set of requests should probably be caching them up front in the install step