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
PLACEHOLDER seems to have been removed?