Open a connection to the easyfire unit.

def _open_connection(self):
        """Open a connection to the easyfire unit."""
        if (self._mode == PROP_MODE_SERIAL):
            self._serial = serial.Serial(self._serial_device, self._serial_speed)
        elif (self._mode == PROP_MODE_TCP):
            self._socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            self._socket.connect((self._ip, self._port))
        elif (self._mode == PROP_MODE_FILE):
            self._file = open(self._file_path, "r")