updated 3-region RATE-OFF-HOLD to 2-region RATE-HOLD.

after calibration, 4 SHORT (200ms on/off) LED flashes == HOLD mode, otherwise == RATE mode. to calibrate the tx for 1500us, apply subtrim to aux1 (with 3P switch in middle position) until at the edge of 4 flashes and no flashes, that is effectively zero stabilization.

aux1 channel pulse width:
1100us-1500us ==> 100%-0% RATE mode
1500us-1900us ==> 0%-100% HOLD mode

in RATE mode, all HOLD correction is reduced to zero
in HOLD mode, all RATE correction is reduced by 50%

eg. if 100% RATE mode = R correction, then 100% HOLD mode = (H + R/2) correction.

TODO: need to tune PID parameters (particularly HOLD mode) or use D direct from rate measurement rather than computing as delta between samples (which themselves are summed over samples!).

deleted all old beta files, need to unify the manual (too many variations). keeping some 20130210_*.hex file for old time's sake.