Added a check to the destructor of Referenced so that it detects referenced

objects that are deleted whilest still having a positive _refCount, such
as when a object has been deleted on the stack, yet other references still
exist for it.  Have put the desctructor implementation in Object.cpp to
avoid adding yet another file with only a couple of lines of code in.

include/osg/Referenced

@@ -6,7 +6,6 @@
 #define OSG_REFERENCED 1
 
 #include <osg/Export>
-#include <set>
 
 namespace osg {
 
@@ -32,7 +31,7 @@ class SG_EXPORT Referenced
         inline const int referenceCount() const { return _refCount; }
 
     protected:
-        virtual ~Referenced() {}
+        virtual ~Referenced();
         mutable int _refCount;
 
 };