AP_Module: added external module hook library

this allows for external modules to be called at defined hook
locations in ArduPilot. The initial example is a module that consumes
the AHRS state, but this can be generalised to a wide variety of hooks

libraries/AP_Module/examples/ModuleTest/ModuleTest.cpp

@@ -0,0 +1,38 @@
+// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+
+//
+// Simple test for the AP_AHRS interface
+//
+
+#include <AP_ADC/AP_ADC.h>
+#include <AP_AHRS/AP_AHRS.h>
+#include <AP_HAL/AP_HAL.h>
+#include <AP_Module/AP_Module.h>
+
+const AP_HAL::HAL& hal = AP_HAL::get_HAL();
+
+// sensor declaration
+AP_InertialSensor ins;
+AP_GPS gps;
+AP_Baro baro;
+AP_SerialManager serial_manager;
+
+// choose which AHRS system to use
+AP_AHRS_DCM  ahrs(ins, baro, gps);
+
+void setup(void)
+{
+    serial_manager.init();
+    ins.init(100);
+    baro.init();
+    ahrs.init();
+
+    gps.init(NULL, serial_manager);
+}
+
+void loop(void)
+{
+    ahrs.update();
+}
+
+AP_HAL_MAIN();