(partly) updated Tagging spec

[Martin Pool]

I've updated the tagging spec to add more use cases, and try to express
them in a way that doesn't assume a solution.

  http://bazaar-vcs.org/BzrTagging

In reviewing that text with Robert & Andrew we had some better ideas
about how to implement them, which I hope will resolve the concerns
people had with various other proposals.  I'm going to add that in now
and will post when it's done.

The basic idea is that tags should be represented by directories in the
filesystem or url namespace, just as branches are.  

-- 
Martin