Added a new flight mode called Acro_Alt_Hold.

Acro_Alt_Hold flight mode combines the roll and pitch rate control from
the Acro mode with Altitude hold of Alt_Hold mode.

Reference:
Making a new flight mode:
http://dev.ardupilot.com/wiki/apmcopter-adding-a-new-flight-mode/
Making it visible in mission planner:
https://github.com/diydrones/MissionPlanner/issues/329

Talha
RI summer intern