Updated Verifiable Claims Use Cases document

Thanks to a bunch of people for chiming in on the use cases.  Most people
agreed that the use case scenarios we had were sufficient (although we were
missing a few - added now).  The biggest set of comments was about
presentation and organization.  So I, with the help of many others, have
reorganized the document to be more about users and how they will benefit.

You can see the latest (and hopefully stable) version at
http://w3c.github.io/webpayments-ig/VCTF/use-cases/

I do not plan to make any further edits to this document prior to the
meeting on 1 July (unless someone points out an embarrassing typo!).

I look forward to your feedback.  Feel free to use the github issue tracker.

-- 
Shane McCarron
Projects Manager, Spec-Ops