WPNav: bug fix to loiter accel calculation

Contribution from Leonard Hall
WPNav: bug fix to reported distance to target

This value is for reporting purposes only
WPNav: replace LOITER_ACCEL_MAX with parameter

Also removed unused _lean_angle_max variable
AutoTest: relax stability patch max distance
Copter: update release notes
Copter: typo in release notes
APM_Control: re-work to allow for rate based control
Plane: added initial implementation of ACRO mode

rate based control with attitude locking
Plane: added ACRO_ROLL_RATE and ACRO_PITCH_RATE parameters

default to 180 degrees/second, which seems reasonable
Plane: added ACRO mode to mode choices
Copter: default Loiter Pos P to 0.8 (was 0.2)
Plane: change to 2.74beta3
Plane: fixed FBWB airspeed control

thanks to Gabor for reporting this!
documentation: minor fix to px4 build instructions (tridge please check)
WPNav: stopping point projection uses wp_leash
Copter: project stopping point for loiter command
Copter: increase Yaw Rate I 0.02
Copter: revert Loiter Pos P to 1.0 (was 0.8)
HAL_PX4: added GPIO driver
AP_Relay: setup pin for PX4 relay support