Versioning strawman

In order to make progress on versioning I think we need a strawman
proposal as a starting point.  Since, I am pleased to observe, there
appears to be a strong minimalist streak on the TC, I would propose the
following, which seems the simplest possible solution.

- The TREX namespace URI includes a version.  For example, it might be
something like http://www.oasis-open.org/trex/namespace/1/0, or it might
be a URN using the FPI scheme as John Cowan suggested.  We will need to
coordinate with OASIS on the exact form.

- TREX elements must use the namespace URI.  The current spec allows a
null namespace URI for TREX elements.  This would no longer be allowed.

- There would be no special forwards-compatibility mechanism.  If a
version 1.0 processor is presented with a version 2.0 stylesheet, then
it will not recognize the namespace URI and will give an error.

James