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 was discussing this point with a colleague and we both ended up agreeing that caching only 200 and 0 responses would be the safest bet for a recommended sample.

It will save people a lot of pain should a bad response get cached.