InertialNav: increase max accel correction to 3m/s
InertialNav: reduce Z-axis time constant to 5
Copter: reduce yaw rate P to 0.2 (was 0.25)
Copter: INS_MPU6K_FILTER defaulted to 20hz
Copter: updated firmware version to 2.9.1b
Copter: bug fix for alt_hold being passed as int16_t to get_throttle_althold_with_slew
Copter: bug fix for throttle after acro flip

Throttle was kept a min if user switched out of ACRO mode while inverted
Copter: update Release notes ahead of 2.9.1b release
Copter: update Release notes again for 2.9.1b release

Missed item re increasing max inav accel correction to 3 m/s
Copter: fix to prevent cli from being entered more than 20seconds after reboot
Copter: ACRO bug fix from Jason
Copter: add V frame support for Quads
Merge branch 'ArduCopter-2.9' of https://github.com/diydrones/ardupilot into ArduCopter-2.9
Merge branch 'ArduCopter-2.9' of https://github.com/R-Lefebvre/ardupilot into ArduCopter-2.9