AP_InertialSensor_MPU9150 I2C userspace driver sensor added.

The MPU9150 is a 9 axis sensor that includes 3 accelerometers,
3 gyroscopes and 3 magnetometers. All accessible through I2C.
The AP_InertialSensor_MPU9150 class allows APM to use this sensor.

Some fixes in AP_Common and HAL_Linux_Class have been made to
support the robot Erle (that wears an MPU9150 at i2c-2).