add VoidEvent

This adds a type of event which can be notified and notifies a listener method without arguments.

It's actually the same patch in this ticket:

http://sourceforge.net/p/poco/patches/110/

An example of how it would be used:

    class Notifier{
    public:
        Poco::BasicEvent event;
            void someMethod(){
            event.notify();
        }
    }

    class Listener{
    public:
        Notifier notifier;
            void setup(){
            notifier.event += Poco::delegate(this,&Listener::listenerMethod);
        }
        void listenerMethod(){
            // do something
        }
    }