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