Adapt the I2C driver to a system clock based wait_for_sample()
implementation.
The sample rate of the sensor has been corrected to 800 Hz (could be
further pushed up to 1KHz).
Filters for the mag have also been created but remain commented until
the code for the mag is ready.
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.
