extensibility -> pxfim

I'm trying to understand whether it makes sense to factor the
extensibility mechanism out of RIF entirely.

As sort of a trial balloon, I've given it a name and thrown together a
skeletal editor's draft, so we can point non-RIF folks at it.

   http://www.w3.org/2007/11/pxfim/

I have no idea at this point where this document will go.  It might well
be discarded as silly, if no one outside of RIF is motivated/interested.
But if there is enough interest from outside of RIF, it might continue
and RIF might be able to divest itself of this technical work.  

      - Sandro