Send a key to the Horizon box.

def send_key(self, key):
        """ Send a key to the Horizon box. """
        cmd = struct.pack(">BBBBBBH", 4, 1, 0, 0, 0, 0, key)
        self.con.send(cmd)

        cmd = struct.pack(">BBBBBBH", 4, 0, 0, 0, 0, 0, key)
        self.con.send(cmd)