'server' command works on Windows
`divshot server` was throwing a `spawn ENOENT` error on Windows.
Windows is detected against `process.env.OS` value, then apply a workaround on the `spawn` call, which delegate its work to Windows `cmd`
Tested on Windows 8.0 and Windows Vista Home, both having `process.env.OS` being 'Windows_NT'.
fix https://github.com/divshot/divshot-cli/issues/118