[OCaml] Add a callback example

It is based on the the Python and Go examples.

Examples/ocaml/callback/example.h

@@ -0,0 +1,20 @@
+/* File : example.h */
+
+#include <iostream>
+
+class Callback {
+public:
+	virtual ~Callback() { std::cout << "Callback::~Callback()" << std::endl; }
+	virtual void run() { std::cout << "Callback::run()" << std::endl; }
+};
+
+
+class Caller {
+	Callback *_callback;
+public:
+	Caller(): _callback(0) {}
+	~Caller() { delCallback(); }
+	void delCallback() { delete _callback; _callback = 0; }
+	void setCallback(Callback *cb) { delCallback(); _callback = cb; }
+	void call() { if (_callback) _callback->run(); }
+};