Add support for Windows (and some other changes)

Our company made a fork of datetime_tz back in 2010, when we added support for windows using win32timezone - which is part of pywin32 - if it is installed.

Unfortunately, we did not immediately attempt to get those changes merged in upstream, and so our version has diverged a little since then, with various minor enhancements.

We realise that this does make for a tricky pull request to handle! Sorry about that... Please let me know if there is anything we can do to make it easier to work with!