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

need to add the following methods to `EventBus`:

``` java
    <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.
