WIP: Python3 support

Preliminary Python3 PR. I ran 2to3, and fixed up some imports and integer division symbols. All tests pass in Python3, hurray!

How we want to handle the Python3 transition is another question.  Personally, I like Python3 so far because it's clearer and stricter in some ways that I find helpful. I'd like to use Python3 for my own work as much as possible, and (assuming it works) use [3to2](https://bitbucket.org/amentajo/lib3to2/overview) to install a Python2-compatible version where necessary as an install-time conversion (or when setting up a Python2 version on PyPi). Of course, [other options are possible](http://docs.python.org/3.3/howto/pyporting.html).