Addd libraryName() method to osg::Object in preperation for the osgDB support

for node kits plugins.

include/osg/Matrix

@@ -34,6 +34,7 @@ class SG_EXPORT Matrix : public Object
         virtual Object* cloneType() const { return osgNew Matrix(); } \
         virtual Object* clone(const CopyOp&) const { return osgNew Matrix(*this); } \
         virtual bool isSameKindAs(const Object* obj) const { return dynamic_cast<const Matrix*>(obj)!=NULL; } \
+        virtual const char* libraryName() const { return "osg"; }
         virtual const char* className() const { return "Matrix"; }
 
         virtual ~Matrix() {}