Add magnetometer capabilities (MPU-9150).

This commit simply adds the possibility to read the raw magnetometer data.
The modifications come from the [sparkfun MPU-9150 library adaptation](https://github.com/sparkfun/MPU-9150_Breakout/blob/master/firmware/MPU6050), here are the concerned original files:
* [MPU9150_raw.ino](https://github.com/sparkfun/MPU-9150_Breakout/blob/master/firmware/MPU6050/Examples/MPU9150_raw/MPU9150_raw.ino)
* [MPU6050.cpp](https://github.com/sparkfun/MPU-9150_Breakout/blob/master/firmware/MPU6050/MPU6050.cpp)
* [MPU6050.h](https://github.com/sparkfun/MPU-9150_Breakout/blob/master/firmware/MPU6050/MPU6050.h)