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've filed https://github.com/GoogleChrome/samples/issues/42 to track the fact that this sample and probably a few others will need to address cache maintenance/versioning. I'll make those changes across all the samples in a consistent fashion in a follow-up PR.

Agreed that in general, pre-fetching will be a best practice, and https://github.com/GoogleChrome/samples/issues/32 will cover more selective caching scenarios. I can envision there being some use cases where blindly caching everything is appropriate, though I'll make it clear in the comments that developers need to clearly understand their use case.