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'm wary of ``standard_library.install_hooks()`` because of what it might do to Plone. Could we switch to the context manager interface instead? http://python-future.org/standard_library_imports.html#context-manager-interface