Web Payments Demo

Hi Ian,

You mentioned in issue #130
<https://github.com/w3c/webpayments/issues/130#issuecomment-220605583> that
you would have liked to see a demo of how payment apps could work. I am
also fond of trying things out for real in order to get a better overview
of how they work or should work, so I've created this
<https://github.com/tommythorsen/webpayments-demo>. It's a chrome extension
which implements payment mediation in desktop browsers. (I have not
implemented the HTTP part of the communication, though, only the proposed
javascript API from my PR).

Along with this extension, there is a primitive payment app which uses the
javascript API to register itself and to process payments, and a simple web
shop which uses PaymentRequest to charge for goods.

All of the source code for the mediator, the merchant and the payment app
is in that github repo. It's still a bit rough around the edges, and I
intend to add more comments and documentation, but I think this could be a
useful platform for trying out different ideas. At least for my own part,
this setup does allow for more rapid prototyping than changing and
rebuilding all of the Chromium source code.

Br,
Tommy