Initializing yaw orientation from magnetometer reading.

- added magnetometer biases (mag_bias/x and mag_bias/y) for hard-iron compensation.
- add two debug topics: imu/rpy/raw and imu/rpy/filtered. imu/rpy/raw can be used for computing mag. biases. imu/rpy/filtered topic is for user readability only. 
- added an option for tilt compensation and initializing yaw orientation from magnetometer reading.
Why don't you also compute `head_z` (eqn 19 in AN4248.pdf)?