adds "callback" and "extend" examples

Examples/perl5/callback/example.i

@@ -0,0 +1,17 @@
+/* File : example.i */
+%module(directors="1") example
+%{
+#include "example.h"
+%}
+
+%include "std_string.i"
+
+/* turn on director wrapping Callback */
+%feature("director") Callback;
+
+/* Caller::setCallback(Callback *cb) gives ownership of the cb to the
+ * Caller object.  The wrapper code should understand this. */
+%apply SWIGTYPE *DISOWN { Callback *cb }; 
+
+%include "example.h"
+