Plane: added a new CRUISE flight mode

See
http://diydrones.com/group/apmusergroup/forum/topics/feature-request-hold-heading-mode

this fixes issue #438
AP_AHRS: removed get_roll_rate_earth() and get_pitch_rate_earth()

these are not used any more
APM_Control: changed attitude controllers to take angular error not angle

this makes it easier for ACRO mode
Plane: update for new APM_Control API
Plane: re-do roll hanging in ACRO mode

this avoids the euler angle zeros of the previous method by using a
gyro integrator on the roll axis

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>
Plane: change to 2.73beta4