Fix for disarm bug - issue #756

Due to baro disturbed by ground effect. Explanation here
https://github.com/diydrones/ardupilot/issues/756

The idea of the fix is we can't leave land while throttle is 0 and then
we ignore the baro and it's disturbances.
Merged Hybrid Mode
Throttle Assist Code
Merge remote-tracking branch 'jason4short/Assist3'
Merge remote-tracking branch 'Ju1ien/Hybrid-Mode'

Conflicts:
	ArduCopter/Attitude.pde