Initializie yaw from calibrated magnetometer data

* Add magnetometer biases (mag_bias/x and mag_bias/y) for hard-iron compensation.
* Initializie yaw orientation from magnetometer reading.
* Add imu/rpy/raw and imu/rpy/filtered as debug topics. imu/rpy/raw can be used for computing magnetometer biases. imu/rpy/filtered topic is for user readability only.
Merge pull request #32 from jackal/mag-fixes

Initializing yaw orientation from magnetometer reading.