Plane: move failsafe variables into a structure

this makes the logic a bit easier to follow
Plane: don't read control switch when ch3 is in failsafe

use ch3_counter not just the final flag
Plane: check the control switch a bit more often

faster response to mode changes
Plane: set mode CIRCLE on short failsafe in AUTO

this makes the code match the docs
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>
Plane: update to new license boilerplate for GPLv3

we had LGPLv2 in some places, despite the fw being GPLv3
AP_Airspeed: added EAS2TAS in airspeed driver

this keeps the true airspeed ratio in the airspeed driver, which seems
the most logical place
AP_AHRS: added true airspeed support in AHRS

use true airspeed for wind calculations, and allow other drivers to
ask for the current ratio
AP_TECS: use ahrs EAS2TAS for true airspeed
Plane: updates for new EAS2TAS scaling
AP_Airspeed: added example sketch for airspeed calibration
Plane: remove old AIR_START code

this was never used or tested
Plane: added SKIP_GYRO_CAL parameter

useful for starting the sim at high altitudes, and for starting planes
without having to hold them still
AP_L1_Control: scale loiter and wp radius with altitude

this should keep the aircraft stable in loiter at very high altitudes

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>
APM_Control: scale controllers for altitude

this allows the attitude controllers to better handle a wider range of
altitudes

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>
AP_Baro: improved the accuracy of the barometer model