Copter: reduce twitch when entering CIRCLE mode

Set loiter target and prev iterations velocity when circle mode is
started
Start circling from projected stopping point
WPNav: add acceleration parameter

WP_ACCEL added to allow user control of acceleration during missions.
Loiter acceleration made to be half of loiter max speed
Copter: update AC3.0-rc6 version and release notes
Copter: reduce default AltHold P to 1.0 (was 2.0)

This helps reduce jumpiness due to althold feed forward
Copter: update firmware version to 3.0.0
AP_InertialSensor: relax accel offset sanity check
Copter: SuperSimple mode bug

SuperSimple bearing was updated without checking for GPS 3D Fix availability.
Copter: calculate home position and bearing when we have 2D Fix
Copter: Fence missing GPS 3D fix check

Fence was missing GPS 3D fix check before selecting recovery decision.
Copter Fence: bug fix to avoid repeatedly invoking LAND mode
Copter: LAND only control horizontal position if we have GPS lock
Copter: update 3.0.1 version and release notes
Copter: GPS Failsafe switch to LAND if circular fence enabled
Copter: change firmware version to 3.0.0-rc1
Copter: double flash arming light when pre-arm checks fail

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.
ACMotors: bug fix to yaw limit
Copter: yaw limit fix to allow I to reduce

We now allow the I term even if we've hit the yaw limits as long as
updating the I term will reduce it
Copter: reduce default Loiter Pos P to 0.2 (was 1.0)
WPNav: reduce default WP_ACCEL and LOITER_SPEED
Copter: relax pre-arm checks for mag field

previously it was 50% ~ 150% of expected mag field but now expanded to
35% ~ 165%.