AP_Math: added two new vector/matrix ops for kalman airspeed filter
AP_GPS: added velocity_vector() method
Plane: added note on type of airspeed in docs
AP_Airspeed: added auto-calibration support

This uses a Kalman filter to calculate the right ARSPD_RATIO at runtime

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>
Plane: use new airspeed calibration code

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>