Open class and Void events

[marcguilera]

Nice little library. I have two suggestions and I could send a PR if it's something you agree with:

1. Make the event class open so it can be extended
2. Make a void event and allow the invoke param to be optional (for param-free type of events)

[svby]

I've addressed the first concern somewhat in the latest release. Additionally, a basic event could be implemented with minimal code by delegating the implementation to a collection instance (see SetEvent.kt)

As for the second, I'm not quite sure how this would fit in with the rest of the library. For a void event, I would just use an Event<Unit> and possibly define an extension function as follows:

operator fun Event<Unit>.invoke() = this(Unit)

This should satisfy the "parameterless" event use-case.