AP_Motors: add get_throttle_out accessor
Copter: add pre_takeoff throttle feedback

pair-programmed with Randy Mackay
Copter: remain landed until throttle output above 25 percent
Copter: integrate pre-takeoff throttle feedback

This feature slightly revs the motors in response to the pilot's input
before takeoff AltHold, Loiter, AutoTune, PosHold and Sport flight modes

pair-programmed with Randy Mackay