Python 3 support

Here's a Python 3 port for your consideration. As of now it passes the tests, but I haven't tried to run it in any real world situations. 

It requires the Python 3 version of repoze.xmliter.

I also removed PasteScript, as it's not ported to Python 3. It needs to be replaced with something, probably gearbox, but things like Paste#urlmap and Paste#proxy needs to be moved over. Or maybe it can be all replaced with mod_wsgi express? 
I'll look into that. I'm currently trying to get the docs running with gearbox under Python 3, though. That will be a separate pull request.