stem.socket.ControlSocket.is_alive() should check if socket is open explicitly

"Checks if the socket is known to be closed. We won't be aware if it is until we either use it or have explicitily shut it down." Instead of this we should do a sample query and if we get a SocketError, then return a False.

def is_alive(self):
  try:
    self.send("GETINFO version")
   except SocketError:
     self._is_alive = False
  return self._is_alive