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/wscript

@@ -0,0 +1,7 @@
+#!/usr/bin/env python
+# encoding: utf-8
+
+def build(bld):
+    bld.ap_example(
+        use='ap',
+    )