fix: changed gevent server to use a real gevent.pool.Pool

The gevent StreamServer requires the pool to really be a gevent.pool.Pool, else the stop() method fails with an AttributeError exception.

What I got before the fix:
Exception in thread Thread-3:
Traceback (most recent call last):
  File "C:\Program Files (x86)\Python\lib\threading.py", line 808, in __bootstrap_inner
    self.run()
  File "C:\Program Files (x86)\Python\lib\threading.py", line 761, in run
    self.__target(*self.__args, **self.__kwargs)
  File "[...].py", line 145, in timeout
    self.server.stop()
  File "E:\Informatik\Python\WebSocket-for-Python\ws4py\server\geventserver.py", line 107, in stop
    _WSGIServer.stop(self, *args, **kwargs)
  File "C:\Program Files (x86)\Python\lib\site-packages\gevent\baseserver.py", line 173, in stop
    self.kill()
  File "C:\Program Files (x86)\Python\lib\site-packages\gevent\server.py", line 87, in kill
    pool._semaphore.unlink(self._start_accepting_if_started)
AttributeError: 'GEventWebSocketPool' object has no attribute '_semaphore'