Initial cut of the prefetch sample.

@kenjibaheux @jakearchibald @slightlyoff

Here's my initial attempt at the prefetch SW sample, closing #29. It could use some additional comments/explanatory text, but I wanted to get some feedback on the actual code.

It uses the native Cache API, currently only available in the latest Chrome Canary build. (Woo! No polyfills!) Since `cache.addAll()` isn't yet implemented natively, it performs multiple `fetch()` + `cache.put()` operations.

You can theoretically see a live demo of this code at https://rawgit.com/jeffposnick/samples/sw-prefetch/service-worker/prefetch/index.html but I'm in the process of figuring out why it doesn't work from there (`TypeError: Failed to fetch` for all `fetch()` requests), while it does work when accessed via http://localhost:8000.
We've been following this format with all of the GoogleChrome samples—driving traffic to chromestatus.com so that folks can read the specs as well as get links to the docs and demos.

Right now there's some extra noise on the chromestatus.com page, but we'll clean it up and ensure that there are links to the definitive isserverworkerready page as well as to the various demos prior to the M40 beta.