Open a serial connection to the arm.

def connect(self, port=None):
        ''' Open a serial connection to the arm. '''
        if port is None:
            self.port = search_for_port('/dev/ttyUSB*', 'ROBOFORTH\r\n',
                                        'ROBOFORTH')
        else:
            self.port = port

        if self.port is None:
            raise ArmException('ST Robotics connection not found.')

        self.ser = r12_serial_port(port)

        if not self.ser.isOpen():
            self.ser.open()

        if not self.ser.isOpen():
            raise ArmException('Failed to open serial port. Exiting.')

        return self.port