Update xbee_reader.py

Added checksum check.
Update xbee_reader.py
Update xbee_reader.py

Checksum verification now perfectly working:
- If CHKSUM is right, the packet is printed out.
<TODO> If not right. request.
Update xbee_reader.py

Added request for retransmission if local checksum doesn't match packet checksum.
Merge pull request #5 from rodkaroma117/develop

Develop