[NewFeature]
--modbus.h
add read coils(0x01) function interface.
--modbus.c
add function code constants.
[REWRITE]add a modbus_proxy to handler the modbus request.
delete extra comments.
1. add request send proxy
2. add response receive proxy
delete unused struct.
callback use a typedef.
add include "modbus_serial.h" for the implement.
remove the extra files.
direct call modbus_serial's function from modbus_proxy.
add modbus_appli as the interface.
add the create function of class.
delete extra include
Merge branch 'master' of https://github.com/wusunjie/open-modbus