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.