Add support for MS Windows

It would be great if Yawf could be used in MS Windows too. It shouldn't be too difficult, the only problematic library used until now is XDG's Base Directory[1]. About the backend, it exists a Cygwin build of Wit too.

ToDo:
- Investigate XDG's Base Directory status in MS Windows, or implement an alternative.
- Give a try to Cygwin's build of Wit, and determine if it's possible to distribute it stand-alone without a full Cygwin environment.
- Investigate how to distribute it: MS Windows executable, installer, etc.

[1] http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html