Copter: bug fix to set sonar_alt_health to zero when disabled
Copter: bug fix for autotrim's roll axis backwards
Copter: THR_MID used to scale manual throttle
Copter: use scaled throttle for accel-throttle's I term

Accel throttle's I term is taken from scaled manual throttle
Copter: manual throttle scaling fix

Scaling of bottom half of throttle was changed from THR_MIN ~ 500.
Previously it was from 0 ~ 500.
Copter: fix up some casting issues for wp_distance
AP_HAL_AVR: Improved AVRTimer micros() and millis()

- More efficient code by using 16-bit timer
- micros() now has proper 1 us resolution and less overhead
- millis() has less overhead
- removed unneeded/unwanted initializatin of timers in AVRTimer::init()

pull request 62, approved and merged by pat