Copter Motors: increased spin-when-armed to 70

Changed choices to be displayed in ground stations
Copter: comment update re low battery event
Copter: update @Range comments for THR_MIN

This should help reduce change of someone setting THR_MIN above 300 (30%
throttle)
PX4: Implement accurate ratiometric voltage reading on Pixhawk

this uses the monitoring of the 5V rail to compensate for changes in
the 5V reference versus the 3.3V ADC reference. It reduces the noise
on ratiometric sensors by a factor of about 6x.
AP_Airspeed: updates for Pixhawk
AP_Motors: fixed example build
AP_Vehicle: added new header for parameters in multiple libraries

this adds a header where we can put vehicle specific parameters that
will be used in multiple libraries
AP_SpdHgtControl: removed AircraftParameters
AP_TECS: changed to AP_Vehicle.h
DataFlash: changed to AP_Vehicle.h
AP_Mount: changed to AP_Vehicle.h
AP_InertialNav: changed to AP_Vehicle.h
AP_HAL_AVR: changed to AP_Vehicle.h
AP_Airspeed: changed to AP_Vehicle.h

also allow updates to ARSPD_RATIO from user while autocal is running
AP_AHRS: changed to AP_Vehicle.h
APM_Control: changed to AP_Vehicle.h
AC_Fence: changed to AP_Vehicle.h
VARTest: changed to AP_Vehicle.h
Plane: changed to AP_Vehicle.h
Copter: changed to AP_Vehicle.h