Update EventBus to add a method to create an EventStream and TopicSubject

[mikebaum]

need to add the following methods to EventBus:

    <T> EventStream<T> getEventStream(TopicId<T> id);
    <T> TopicSubject<T> getTopicSubject(TopicId<T> id);

and a new class TopicSubject which extends EventStream adding the publish method.