Auto Takeoff

Proof of concept for Auto-takeoff. Needs proper setting of target altitude, and a param for takeoff height.
Takeoff is 2.5m by default for testing.
To test with RC - set takeoff to a swicth position. raise throttle above zero before entering mode or it will not engage. This is a safety feature until the tablet interface is sorted. The mode is not intended to be used by an RC in the final implementation.

Conflicts:
	ArduCopter/defines.h