Preparing wiring-pi 2.0.0

- Split the source code (based on libWiringPi source code hierarchy)
- ADD: Implement I2C functions
- ADD: Aliasing serialPrintf to serialPuts
- FIX: missing constants in pinMode check
- ADD: missing constant PI_MODEL_COMPUTE
- ADD: wiringPiISR support (experimental)
- UPD: Exports constants from C++ (ReadOnly | DontDelete)
- UPD: Exports setup function from C++
- UPD: setup function no longer accepts calling without mode defined
(breaks backward compatibility)
- ADD: Better types and allowed values check
- ADD: Better error messages
- UPD: wiringPiSPIDataRW now takes two parameters, a node buffer as
second parameter (instead of a string as second and length as third)
- UPD: README.md (move documentation to DOCUMENTATION.md)
- UPD: Documentation
- UPD: Package.json version and contributors section
- ADD: version history file (CHANGELOG.md)
- DEL: Backward compatibility constants