allow call-backs when a tool is a created or retrieved 
class myObserver : public ITooSvc::Observer {
         cout << 
"tool " << tool.
name() << 
" created " << 
endl;
     }
         cout << 
"someone requested " << tool.
name() <<
endl;
     }
}
myObserver *observer = new myObserver;
IToolSvc implementations will call 'Observer::onCreate' when a tool is created, and 'Observer::onRetrieve' if a tool is retrieved.
The user is responsible of the life time of the object and must un-register it before it is deleted (e.g. during the finalization). 
Definition at line 223 of file IToolSvc.h.