SW Sample: Post Message

@gauntface @jakearchibald @kenjibaheux @coonsta @slightlyoff & other interested parties:

This sample demonstrates the use of `postMessage()` to interact with the service worker controlling a page. Users can add, delete, and get a list of items in the cache.

There's a live demo at https://rawgit.com/jeffposnick/samples/sw-post-message/service-worker/post-message/index.html