Support running on MS Windows

[clue]

MS Windows is not officially supported by Zenity, yet several (unofficial) builds are available online.

Among those that appear to be more mature is this one: http://opensourcepack.blogspot.de/2013/02/zenity-360-portable.html
Its feature set is limited and it doesn't support all of Zenity's options.

Depends on #5 to replace icecave/mephisto with react/child-process.

[clue]

The above experimental build works reasonably well.

Either put the zenity.exe in your project root folder (where you invoke php), put it into your $PATH or explicitly define the path via Launcher::setBin().

Please note that the underlying component used to launch processes still has some compatibility issues on Windows (reactphp/child-process#9).

There's not much we can do here, so I'm considering this done.