Py3.3 support for xlwt

Hi John,

I have patched xlwt to support Py3 as well as Py2.7 with a single codebase (i.e. no embedded 2to3 conversion). It may also support Py2.6, but if not I can probably add Py2.6 support easily too.

This currently introduces one dependency: my "python-future" package for Py2/3 compatibility. However, very little from the "future" module is actually needed now, after conversion help with the "futurize" script, so this dependency can probably be removed easily enough, if a few functions and decorators from it are merged into xlwt.

I would be interested in your feedback. The patch set is big but most of the changes should be uncontroversial (e.g. converting print statements to print functions).

The unicode / bytes disambiguation was the most difficult part. It seems to work now under both Py3 and Py2 for all the example scripts, but this could do with more testing.

Kind regards,
    Ed Schofield (ed@pythoncharmers.com)