Mock responses example

@gauntface @jakearchibald @kenjibaheux @coonsta @slightlyoff & other interested parties:

Here's a sample that returns mock responses to API requests (assuming a specific request header is set), closing #34

You can view a live version of the sample at https://rawgit.com/jeffposnick/samples/sw-mock-responses/service-worker/mock-responses/index.html