Locate Lost Copter with Stick Combo when disarmed

This will start beeping the buzzer when holding elevator back and aileron right when throttle is down and copter is disarmed.

Name can be discussed. FAILSAFE_LOCATE_COPTER, LOCATE_ME_ALARM,FAILSAFE_LOSTMODEL, FIND_MY_COPTER, +++

Feature is disabled by default since i'm unsure about the size of the feature and space on the APM 2
it will also send a message to the ground station saying that the alarm is enabled.

I would like to implement a mavlink message to set this off also but i haven't looked at that yet.

LED flashes should also be implemented and some way of triggering the AP_notify flag when the copter is entering failsafes and lands without RC or ground station telemetry. Which LED Should we use for this?

This is related to issues:
https://github.com/diydrones/ardupilot/issues/291
https://github.com/diydrones/ardupilot/issues/1119
https://github.com/diydrones/ardupilot/issues/441

it have been tested on APM2 and SITL