Partial Python implementation of TREX

During a couple of transatlantic flights last week I was able to knock off a
decent amount of a Python implementation of TREX. I haven't looked at James
Clark's implementation at all so this is completely clean-room.

I'm really happy with TREX so far from an implementation point of view
although I obviously started with the easier parts. I've implemented those
features described in sections 1 through 3 of the tutorial, almost done 4
(grammar/start/define/ref).

It'll be up at pytrex.sourceforge.net real soon now, but feel free to email
me privately if you'd like me to send you the python before then.

I've also been building up test cases as I've implemented features and I'll
be making those available too.

James Tauber