Minimum airspeed for throttle engagement on auto-takeoff

Adds enhancement requested in issue #81 as per discussion on http://diydrones.com/forum/topics/auto-takeoff-sequencing-problem

Fix is to add a new parameter that specifies a minimum GPS ground speed that must be exceeded before throttle is engaged. Default is at 0 to maintain current default behaviour of instant-start when stationary.

As this has some safety implications the default value might be better set at a value greater than 0, but this may also introduce bad behaviour at low values where the throttle may start intermittently with bad GPS readings.

This also introduces a strict dependence on GPS lock for auto-takeoff which may be a good thing.