disable TIMER0, lock-free rx_in copy, servo_frame sync

= disable TIMER0
- replace micros()/delay() with micros1()/delay1() based on TIMER1
- lock-free micros1()
- thus, servo jitter is now further reduced (about +/- 250ns on my scope)

= lock-free method to copy isr-owned vars to main thread.

= sync rx_in frame with servo_out frame.
- servo frame is emit once ref rx channel is received in the common case.
- currently, the sync is at the frame level, next todo would be at the channel level.

= added 2 hex files for this build
- RX3S_V1_20130113
- RX3S_V2_20130113
bunch of bug fixes and small cleanups.
- timer1 rollover. now returns full 32bit result in us.
- use if ((int32_t)(now - then) > duration) for scheduling test.
- check device id before proceeding.
- refactored PCINT0/2 a little to reduce interrupt-disabled interval.
- start_servo_frame() set up the condition for COMPA interrupt instead of calling the ISR directly.
- schedule the reading of IMU immediately after starting the first servo pulse (to synchronize i2c access with servo - but i think not really necesary).

- describe detail about "neutral position" in README.md.
fixed very VERY rare corner case on possible race between timer1 ISR and micros1() over timer1_ovf

let rx3s use 400khz i2c, but leave mpu6050 with 100khz
- added build files
Merge remote-tracking branch 'remotes/origin/timer0_servo_part1'