'server' command works on Windows

`divshot server` was throwing a `spawn ENOENT` error on Windows.
Windows is detected against `process.env.WINDIR` existence.
We then apply a workaround on the `spawn` call, which delegate its work to Windows `cmd`
Not tested on other platforms than Windows 8.0.
fix https://github.com/divshot/divshot-cli/issues/118