Finish making the code Python 3.0 compatible.

[rec]

There has been quite a lot of work done on this - in particular, we have the correct headers on each file, we already use print() the function everywhere, we use the six library https://pypi.python.org/pypi/six/ in many places.

This got stopped because pi3d isn't Python 3 compatible, but now it is. It should be pretty straight-forward to take this process up again and complete it.