Add a sw-helpers-based demo

[jeffposnick]

It would be good to use this repo to host a few other demos, including one based on https://github.com/GoogleChrome/sw-helpers/

I'm going to port over https://github.com/GoogleChrome/sw-precache/tree/master/app-shell-demo to use the new libraries and try to find a place for it here, which might mean rearranging some of the other files to structure things so that each demo is independent.

CC: @addyosmani @gauntface