Python 3 support

Following discussion on PR #32, this adds Python 3 support without any external dependencies. Python 2.6 and above and Python 3.3 and above should work; older versions will not.

The tests are passing on Python 2 and 3; I haven't tried the examples.
I've reverted some of my changes, but I'm not inclined to spend much time on it, since as you say, the best thing is probably to rip it out. I don't know what it should be replaced with, though. It's used in the ExcelFormula* code.