[rotorcraft] CAUTION! stabilization int_quat ref model scaling fixes

Now the attitude reference should finally have the correct scaling.

For end users that means it is not so sluggish on RC input anymore and you should get a snappy reference.
CAUTION! Test fly carefully, you might need to lower the gains.
[rotorcraft][stabilization_ref_quat] saturate rate and trim accel
[conf] stm32f4 discovery test board files
[conf] updates for fraser quad with spektrum
[rotorcraft] PRINT_CONFIG_VAR for GUIDANCE_V_NOMINAL_HOVER_THROTTLE
[lpc21] only indentation cleanup
[boards] stm32f4_discovery board file for some testing
[rotorcraft] v_adapt: limit min/max estimated hover throttle

The adaptive vertical filter estimates the ratio of vertical acceleration over thrust command.
In effect this will now limit the throttle applied in hover between 20% and 75% of max throttle by default.
[rotorcraft] call ahrs_update_gps

This makes AHRS_USE_GPS_HEADING usable on rotorcraft firmware as well.
Thanks Pranay.
[fixedwing] additonals for energy ctrl

better handling for engine out and max_acc can be set in airframe
closes #497
[modules] fix to use amsys airspeed for control
[modules] a little airspeed_amsys cleanup
[mcu_periph] replace gpio_output_[high|low] with gpio_[set|clear]

That means using the already existing gpio_set and gpio_clear from libopencm3 for stm32.
And defining these for the lpc.
set paparazzi_version to v5.1_devel
[modules] angle of attack sensor fix and cleanup
[dox] update comment in state inteface
improve paparazzi_version if git tags are not found
Merge pull request #498 from flixr/gpio_setclear

[mcu_periph] replace gpio_output_[high|low] with gpio_[set|clear]
KroozSD imu driver improvements

Fixed magnetometer axes.
Accel median filter disabled by default.

closes #501
[STM32] Very limited simple SPI1 slave driver

closes #500