Revised PID engine and setup to combine RATE and HOLD.

integrating rate over fixed time steps derives attitude, so the sum_err in the pid state with rate as the control variable represents the amount of attitude change since it was last set to zero. multiply that by the I parameter derives the correction due to attitude change. reseting relative attitude simply clears sum_err to zero.

- new compute_pid(), separate parameters from state.
- 2 sets for pid parameters: RATE and HOLD
- RATE is PD
- HOLD is PID with reduced P (and also D for now)
- reset attitude in HOLD mode by clearing sum_err in pid state to zero

generated hex files, removed previous 20130420_* and test hex file..

TODOs:
- re-evaluate the PID relative weight shifts, i think PID_KI_SHIFT needs to be lower (from 6 to 4)
- re-evaluate the D parameter in HOLD pid
Merge branch 'HH' of https://github.com/noobee/FlightStab into HH