Py3.3 support for xlwt

Hi John,

I have patched xlwt to support Python 3.3 as well as Python 2.6/2.7 with a single codebase (i.e. no embedded 2to3 conversion).

This currently introduces one dependency: my <a href="http://python-future.org">future</a> package for Py2/3 compatibility.

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 Python 3.3 and Python 2.7 for all the example scripts, but this could do with more testing.

Kind regards,
    Ed Schofield (ed@pythoncharmers.com)