Add Java 8 lambda-friendly factory method to EventSource

From Java 8 this allows:
```java
EventSource.whenConnected(es -> clock.tell(es, null));
```
in place of:
```java
new EventSource() {
    public void onConnected() {
        clock.tell(this, null);
    }
}
```

If this is useful then similar factory methods can be added for Comet and Websocket, and possibly other classes that are usually implemented with anonymous classes but are not functional interfaces.