liblightmodbus v2.0

Changes:

• Start of semantic versioning era (this would have been version 1.4)
• Documentation is now generated by Doxygen, and available online
• Replaced the shell+makefile build system with CMake
• Added support for user-defined functions
• Added support for user register/coil callback functions
• Improved the error handling system
• Replaced #define based types with proper enums
• Add option to disable master's processed data buffer
• Add option to enable invasive response parsing in master device
• Macro based wrapper functions replaced with static inline functions
• Endianness handling improvements

PPA is available here.