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: add @Increment to PID parameters descriptions