Update to libWiringPi v2

- Update to libWiringPi v2
- Move values check into c++ functions
- Update documentation
- Add support for MCP23S17 (mcp23s17Setup)
- exports.js dynamically exports functions from c++ module
- Add support for Physical numbering scheme in setup function
- Reorganise and add some libWiringPi constants
- Added functions: wiringPiSetupPhys, pinModeAlt, analogRead,
analogWrite, wpiPinToGpio, physPinToGpio, setPadDrive, getAlt,
digitalWriteByte, pwmSetMode, gpioClockSet, mcp23s17Setup
- Update package.json (version, contributors)
- Add a lot of comments in source code