[imu] use OrientationRep for body_to_imu
[settings] add settings to change BODY_TO_IMU
[imu] possibily to set imu_to_body to current roll/pitch and reset to airframe values
[imu] move settings handlers to c file
[settings] rename imu_to_body to body_to_imu.xml
[imu] imu_SetBodyToImuCurrent: only adjust by current roll/pitch

so you can use it if initial body_to_imu is not zero and also use it repeatedly
[imu] imu_to_body settings: combine set current and reset in one
Merge pull request #783 from flixr/body_to_imu

[imu] body_to_imu: use generic orientation representation and expose it in settings to change during operation

You can now change the body_to_imu orientation via settings from the GCS, which should make experimentally "tuning" this for nice hover easier.
Also added setting "b2i curr roll/pitch" to set body_to_imu to current roll/pitch (on TRUE) and reset it to airframe file values on FALSE.