We now allow the I term even if we've hit the yaw limits as long as
updating the I term will reduce it

Hardly my finest work but it's temporary because we're going to replace
all the led, buzzer etc with a library called AP_Notify in the near
future.

Fence was missing GPS 3D fix check before selecting recovery decision.

SuperSimple bearing was updated without checking for GPS 3D Fix availability.

This helps reduce jumpiness due to althold feed forward

WP_ACCEL added to allow user control of acceleration during missions.
Loiter acceleration made to be half of loiter max speed

Set loiter target and prev iterations velocity when circle mode is
started
Start circling from projected stopping point

Verify_takeoff was always returning false if throttle was zero.  The
pilot should be able to move their throttle back to zero immediately
after initiating the mission

the choice of analog source is now in the airspeed driver

this gives us support for arbitrary analog pins for the airspeed
sensor, plus support for the EagleTree airspeed driver on PX4

This bug affected parameters where the defaults are overridden in the
object constructor. For example, a PID object may have a default value
for PID_D of 0.0, but have a constructor based default of 0.2. If the
user tries to set the value to exactly 0.0, then the set wouldn't happen,
as the value matches the value in the object default var_info[]
table. 

This change ensures we force a save to eeprom if the value is changing
from the current value, regardless of the var_info[] default.

Thanks to Tom Coyle for finding this bug!

just needs a wrapper driver in APM now