Portability Requirement

As per my action item today, I have included a requirement and scenarios
about portability of claims.  This has been checked in and pushed to [1].
Please take a look and let me know if it captures what we discussed!

[1] http://w3c.github.io/webpayments-ig/VCTF/use-cases/

P.S.  I also changed "curator" to "repository" as per the poll we took.
Since this is in an external file, your browser may cache the content.  If
you see dfn link errors, that's what it means.

-- 
-Shane