[DESIGN] Enable support for Up/Down casting of events

[yreynhout]

Meet a new citizen

    public interface IEventConverter<in TEvent> 
    {
        IEnumerable<object> Convert(TEvent @event);
    }

Requires that the IEventDeserializer relaxes a bit as well

    public interface IEventDeserializer
    {
        IEnumerable<object> Deserialize(ResolvedEvent resolvedEvent);
    }