Phase 1 of HOLD mode changes.

as discussed at http://www.rcgroups.com/forums/showpost.php?p=25052777&postcount=1008

- enable stick-controlled roll rate (ie. stick position sets commanded rotation rate)
- retain stick-controlled gain (ie max stack attenuates correction to zero)
- previous two factors "oppose" each other. as stick position increase, commanded rate increases by axis gain also reduces.

- activate by setting HOLD mode (HOLD = "RATE with stick controlled rotation rate" during this transition)
- thus, reset HOLD PID parameters to be same as RATE at this time.
stick-controlled gain uses the VR direction to set polarity of setpoint.

the stick position (direction and magnitude) is used to set the pid setpoint. need to set the polarity to be canceled out by the VR polarity later. note that there might be an issue with vr magnitude is close to zero, so the sign can switch between +1 and -1, but since the VR magnitude is also close to zero, the correction will be scaled down to zero anyways.
Phase 2 of HOLD mode changes

- pid param has non-zero Ki (500 default)

- per-axis i_limit parameter (was renamed from i_windup). i_limit[axis] sets the limit of sum_err[axis], which is the attitude error, in the pid controller.

- set max stick-controlled rotation rate to "98 deg/s". in attitude hold mode, the correction due to attitude error is dominant, we need the stick-controlled rotation rate input to introduce the attitude error by accumulating/integrating the rotation rate to derive rotation angle. the balance is between the a) the effect of correction += Ki*sum_err vs b) sum_err += err, where err is the rotation rate. (b) seemed too fast at "391 deg/sec", so it's reduced to "98 deg/sec" by advanced hand waving (ie. guessing).

- btw, i'm not sure if i'm reading the itg3205 datasheet right, in that 32768 == 2000 deg/sec rotation rate. would like to hear any feedback about this.

- finally, added a deadband filter for the VR POT so any reading < 8 (in magnitude) is reduced to 0.
reverted two files deleted during the merge from hold branch to main branch.