Selective caching example

@gauntface @jakearchibald @kenjibaheux @coonsta @slightlyoff & other interested parties:

Here's a sample covering selective caching of responses, closing https://github.com/GoogleChrome/samples/issues/32

In this implementation, I'm pulling in a Google Web Font onto a controlled page, and within the service worker's `fetch` handler, caching responses that have `Content-Type: font/...` set. While not particularly useful on its own, I think it illustrates how developers can control what type of responses they want to cache.

Additionally, this is the first sample that uses a cache naming/versioning scheme along with an `activate` handler that clears out old caches. Any feedback about how I implemented that would be appreciated. If it looks good, I'll go back and add in a similar approach to the existing samples, closing https://github.com/GoogleChrome/samples/issues/42