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
I talked to @slightlyoff about this, and he was (somewhat?) surprised that opaque filtered responses always had a `status` of 0, and suggested that I file https://github.com/whatwg/fetch/issues/14 to see whether that restriction could be relaxed in the `fetch()` spec.

I'll keep caching whenever `response.status < 400` and make a point of specifically highlighting this tradeoff in the comments. As long as the `fetch()` spec stays the same, it'll be each developer's decision whether they want to err on the side of caching too much or too little.