AP_AHRS: Added getters for compass and GPS.
AP_Arming: First commit of AP_Arming library.

Intended to prevent unintentional motor startups.
Plane: Now using AP_Arming library.
Plane: when no GPS lock always send GPS messages
AP_Arming: fixed build
Plane: fixed arming bit in MAVLink heartbeat message
AP_Arming: fixed public/private split
Plane: small arming tidyups
AP_Arming: make it possible to change ARMING_REQUIRED without a reboot
build: fixed a build cmp warning
AP_Airspeed: avoid a warning
AP_GPS: GPS week is unsigned
AP_InertialSensor: reduce the number of format string warnings