Handle MPU temperature as float instead of int
libpixi: add pixi_i2cReadWrite() function

Supports combined write-read.

To simplify using this function, introduce struct I2cDevice with
pixi_i2cOpen2() and pixi_i2cClose().
libpixi: use pixi_i2cReadWrite() to read MPU registers
pio: add i2c-transfer command

Provides combined write-read facility.
libpixi: add pixi_i2cMultiOp function

Provides a simple interface for running a sequence of read and/or write
operations.
libpixi: extend MPU API

Improve code for reading registers.

Add simple functions for reading sensors:
int pixi_mpuReadAccel (MpuAxes* axes);
int pixi_mpuReadGyro (MpuAxes* axes);
int pixi_mpuReadMotion (MpuMotion* motion);
pio: use improved MPU API
libpixi: add some i2c trace logging
libpixi: add functions to get/set mpu gyro/accel scale
pio: add commands to get/set mpu gyro/accel scale
pio: improve handling of mpu units

Change mpu-monitor-motion to display values in units of g and dps.
pio: add many mpu read commands

mpu-read-temp         Read MPU temperature (degrees C)
mpu-read-acc          Read accelerometer (g)
mpu-read-acc-x        Read accelerometer x value (g)
mpu-read-acc-y        Read accelerometer y value (g)
mpu-read-acc-z        Read accelerometer z value (g)
mpu-read-gyro         Read gyroscope (dps)
mpu-read-gyro-x       Read gyroscope x value (dps)
mpu-read-gyro-y       Read gyroscope y value (dps)
mpu-read-gyro-z       Read gyroscope z value (dps)
libpixi: rename pixi_i2cReadWrite to pixi_i2cWriteRead
libpixi: add pixi_isLocaleEncodingUtf8()
libpixi: add LIBPIXI_PRECONDITION_MSG
libpixi: add MPU magnetometer support
pio: add many MPU magnetometer commands

mpu-monitor-mag
mpu-read-mag
mpu-read-mag-x
mpu-read-mag-y
mpu-read-mag-z
mpu-read-mag-id
libpixi: add a flag for hiding commands/groups
pio: hide uart-test, uart-rw-monitor and mpu-read-mag-id commands
pixi-tools-0.6