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.
Previously on Python 2: `error: argument for 's' must be a string`.

There is no code in xlwt accessing it with anything besides the default value (`None`). I don't have Excel handy, and I wouldn't be sure where to look for this value in Excel in any case.

I'm confident it will work for ascii names. For non-ascii, I can't see any practical way for me to establish what is right until someone runs into a problem.