Add flow diagrams to the spec

[stpeter]

As discussed in the 2018-09-11 task force call, it would be helpful to add flow diagrams to the spec. In IRC @ianbjacobs pasted the following links:

http://www.plantuml.com/plantuml/proxy?fmt=svg&src=https://www.w3.org/2018/08/tokenization-flows/sameph.pml

http://www.plantuml.com/plantuml/proxy?fmt=svg&src=https://www.w3.org/2018/08/tokenization-flows/anyph.pml

http://www.plantuml.com/plantuml/proxy?fmt=svg&src=https://www.w3.org/2018/08/tokenization-flows/backend.pml

[ianbjacobs]

I have added two diagrams to the specification (section 13)
https://w3c.github.io/webpayments-methods-tokenization/index.html#flow

(Right now they don't render as images but you can follow the links to get the images; I have not yet figured out why the SVG is not rendering as an image.)