proposed change to UC & Req doc to use issue numbers from tracker

Hi-

I've pushed a change to `jtandy-uc-edit` branch which uses in-built
functionality in respec to link Issue class blocks directly to the issues
in the tracker. The key point is that the numbers don't change when you add
new issues to the document.

See
https://rawgit.com/w3c/sdw/jtandy-uc-edit/UseCases/SDWUseCasesAndRequirements.html
for what this looks like.

If this is desirable, please merge to gh-pages.

Jeremy