plugins: imu_pub: Add RAW_IMU, SCALED_IMU and SCALED_PRESSURE handlers

Fix #13. Refactor message processing.

Combination of used messages:
On APM: ATTITUDE + RAW_IMU + SCALED_PRESSURE
On PX4: ATTITUDE + HIGHRES_IMU
On other: ATTITUDE + (RAW_IMU|SCALED_IMU + SCALED_PRESSURE)|HIGHRES_IMU

Published topics:

 * ~imu/data         - ATTITUDE + accel data from *_IMU
 * ~imu/data_raw     - HIGHRES_IMU or SCALED_IMU or RAW_IMU in that order
 * ~imu/mag          - magnetometer (same source as data_raw)
 * ~imu/temperature  - HIGHRES_IMU or SCALED_PRESSURE
 * ~imu/atm_pressure - same as temperature