AP_AHRS: added get_yaw_rate_earth()

used to estimate course correction on takeoff
Plane: make auto takeoff independent of compass

this solves a problem of poor initial yaw due to poor compass offsets
causing a takeoff to not be in the direction the plane is pointing. A
summed gyro is used until the GPS speed is above 5m/s for 2 seconds,
then the GPS heading corrected by the summed gyro error is used for L1
based navigation for the rest of the takeoff