Incorporate Head IMU

[jgueldenstein]

The head should receive it's own IMU to reduce the calibration effort.
Integrating this is not trivial as we need to decide how:

1. we could introduce an additional link that inherits the position from the head link and the roll and pitch from the imu measurement
2. we could overwrite the transformation from neck to head, this would require to modify the configuration for the robot state publisher