Feature py3k, initial support for atom on python 3

Hey Chris, 

As discussed, here's initial support for python 3, runs fine on os x and gentoo (no ability to test Windows here :). Needs proper str/unicode consistency due to your custom PyStringMaker (didn't want to rip that out without checking), PySequenceMethods needs discussing, and i need to fix sortedmap, but the rest looks ok.

Cheers,
Dave