1. Add Readme for simple instruction of this foler
2. delete unnessary lines of code in test.cpp and test_sender.cpp to make then clear and self-explanatory
3. delete XBEE_msg::_API_type, because it's duplicate with the first byte of the _frameptr
4. Allocate memory automatically for XBEE_msg::_frameptr when the constructor of XBEE_msg is called, the memory size is defaultly set to be 512 bytes
5. Add comments in XBEE.h and XBEE_msg.h in case someone want to read it