Service-worker-based payment app proposal

I have completed my proposed edits to the payment app specification, and 
submitted a PR against the spec:
https://github.com/w3c/webpayments-payment-apps-api/pull/41

This proposal is based on the conversation in the following issue:
https://github.com/w3c/webpayments-payment-apps-api/issues/33


For ease of review, you probably want to read the rendered version in my 
fork:
https://adamroach.github.io/webpayments-payment-apps-api/

-- 
Adam Roach
Principal Engineer, Mozilla