websockify: use threading instead of multiprocessing on Windows

multiprocessing isn't able to copy the socket on Windows: "AttributeError: 'module' object has no attribute 'fromfd'"

With kanaka:master: 

    C:\Users\meta\Documents\noVNC\utils>\python27\python .\websockify   1111 192.168.56.102:5900  --verbose
    Traceback (most recent call last):
      File ".\websockify", line 1, in <module>
        websockify
    NameError: name 'websockify' is not defined

After `cp websockify websockify.py`:

    C:\Users\meta\Documents\noVNC\utils>\python27\python .\websockify   1111 192.168.56.102:5900  --verbose
    WARNING: no 'numpy' module, HyBi protocol is slower or disabled
    WARNING: no 'resource' module, daemonizing is slower or disabled
    WebSocket server settings:
      - Listen on :1111
      - Flash security policy server
      - No SSL/TLS support (no cert file)
      - proxying from :1111 to 192.168.56.102:5900

      1: 127.0.0.1: new handler Process
    WARNING: no 'numpy' module, HyBi protocol is slower or disabled
    WARNING: no 'resource' module, daemonizing is slower or disabled
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "C:\python27\lib\multiprocessing\forking.py", line 381, in main
        self = load(from_parent)
      File "C:\python27\lib\pickle.py", line 1378, in load
        return Unpickler(file).load()
      File "C:\python27\lib\pickle.py", line 858, in load
        dispatch[key](self)
      File "C:\python27\lib\pickle.py", line 1133, in load_reduce
        value = func(*args)
      File "C:\python27\lib\multiprocessing\reduction.py", line 193, in rebuild_socket
        _sock = fromfd(fd, family, type_, proto)
      File "C:\python27\lib\multiprocessing\reduction.py", line 182, in fromfd
        s = socket.fromfd(fd, family, type_, proto)
    AttributeError: 'module' object has no attribute 'fromfd'

threading seems to work fine with two concurrent connections.