add VoidEvent

[arturoc]

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
    }
}

[aleks-f]

Skimming over it, it looks good. Would you mind adding a test case before I merge it? Thanks.

[arturoc]

i've added the tests in the classes that test the other kind of events, let me know if that's ok.

the other commit has the Priority version of the void event and all the needed classes, before there was only FIFO and Basic