AP_Motors: throttle_pass_through accepts pwm
Copter: move esc calibration to separate file

Restructured into case statement
Replaced use of g.throttle_max with definition
Added more comments
Send message to ground station instead of printing on console (although
probably both are unlikely to be read)
Copter: auto esc calibration

Throttle is raised to maximum and then lowered without any input from
pilot
Copter: add LOG_FROM_STARTUP definition

Uncommenting this line in APM_Config.h will start logging as soon as the
board starts up instead of waiting for the vehicle to be armed
Copter: increase Alt Disparity check to 2m
Copter: cleanup enabling of cli and frsky telem for APM
Copter: low battery msg to severity high
LowPassFilter: add div by zero check