libpixi: add reserved fields to various public structs

Reserved for future use.
libpixi: rename pixi_pixiGpio* functions to pixi_gpio
libpixi: take care of GPIO initialisation internally
libpixi: hide pixi_gpioSetMode()
libpixi: be explicit about using the wiringPi GPIO mapping

Avoid using the functions which implicitly map the GPIO pins numbers.
libpixi: remove Pi GPIO functions which implicitly map pin numbers
libpixi: use larger values for log level constants
Move some i2c from pio to libpixi
libpixi: add util/bits.h
Add initial MPU support
python: fix some references to obsolete GPIO functions