1. Implement bool verify_CheckSum() for XBEE_msg,
	please call this when you need to verify the CheckSum of the recevied RF packet
2. Begin to implement the XBEE_send_msg() function for sending a XBEE_msg.
  The XBEE_msg should will well initilized, with specified destination address and maybe other details, the functionality of XBEE_send_msg() would be just to detect possible escape characters in the XBEE_msg and send the XBEE_msg to the serial port.