Features
* check byte count in message
* args in command line (master/slave)
* broadcasting
* slave must listen only request sent for him

Cleanups
* use uint16_t, reduce size of structs (e.g. address is uint16_t)
* split compute_response_size to reuse
* split modbus.c (tcp/rtu, query/response)
* t_id in param_msqg
* only one build_header_message function to replace build_query_packet/response_packet

Documentation
* README with a example to test the library
* Better API coverage in source code