Added a DeleteHandler into osg::Referenced so that it can delete objects

via a seperate delete handler.  Useful for making unref()/ref() thread safe if
the users needs to address this issue.

src/osg/Referenced.cpp

@@ -0,0 +1,38 @@
+#include <osg/Referenced>
+#include <osg/Notify>
+#include <typeinfo>
+
+namespace osg
+{
+
+static std::auto_ptr<DeleteHandler> s_deleteHandler(0);
+
+void Referenced::setDeleteHandler(DeleteHandler* handler)
+{
+    s_deleteHandler.reset(handler);
+}
+
+DeleteHandler* Referenced::getDeleteHandler()
+{
+    return s_deleteHandler.get();
+}
+
+
+#ifdef OSG_COMPILE_UNIT_TESTS
+int Referenced::_createdCount = 0;
+int Referenced::_deletedCount = 0;
+#endif
+
+Referenced::~Referenced()
+{
+    if (_refCount>0)
+    {
+        notify(WARN)<<"Warning: deleting still referenced object "<<this<<" of type '"<<typeid(this).name()<<"'"<<std::endl;
+        notify(WARN)<<"         the final reference count was "<<_refCount<<", memory corruption possible."<<std::endl;
+    }
+#ifdef OSG_COMPILE_UNIT_TESTS
+    _deletedCount ++;
+#endif
+}
+
+}; // end of namespace osg