Instituting Yaw Differential on Multirotors. Intent is to stop "rise on yaw input".

Since motors with increasing speed due to yaw input seem to generate more thrust that motors that slow lose thrust, thus net thrust goes up, causing copter to climb.
Values are a guesstimate, proven out by test flying.  This could probably become a parameter.

libraries/AP_Motors/AP_MotorsMatrix.cpp

@@ -125,6 +125,7 @@ void AP_MotorsMatrix::output_armed()
 	int8_t i;
 	int16_t out_min = _rc_throttle->radio_min;
 	int16_t out_max = _rc_throttle->radio_max;
+	int16_t yaw_contribution = 0;
 
 	// Throttle is 0 to 1000 only
 	_rc_throttle->servo_out = constrain(_rc_throttle->servo_out, 0, _max_throttle);
@@ -141,10 +142,16 @@ void AP_MotorsMatrix::output_armed()
 	// mix roll, pitch, yaw, throttle into output for each motor
 	for( i=0; i<AP_MOTORS_MAX_NUM_MOTORS; i++ ) {
 		if( motor_enabled[i] ) {
+			yaw_contribution = _rc_yaw->pwm_out*_yaw_factor[i];
+			if (yaw_contribution > 0 ){
+				yaw_contribution *= 0.7;
+			}else{
+				yaw_contribution *= 1.42;
+			}
 			motor_out[i] = _rc_throttle->radio_out +
 							_rc_roll->pwm_out * _roll_factor[i] +
 							_rc_pitch->pwm_out * _pitch_factor[i] +
-							_rc_yaw->pwm_out*_yaw_factor[i];
+							yaw_contribution;
 		}
 	}