v0.3.0

• BREAKING CHANGE: Changed and simplified the EventBus API. We can now dispatch
  any Dart object as an event. Before, we had to create an EventType for every
  type of event we wanted to fire. Now we can use any class as an event.
  Listeners can (optionally) filter events by that class.
• Added a way to create a hierarchical event bus that filters events by
  class and their subclasses. This currently only works with classes
  extending other classes and not with implementing an interface.
  We might have to wait for
  https://code.google.com/p/dart/issues/detail?id=20756 to enable interfaces.
• BREAKING CHANGE: Default to async mode instead of sync. This now matches the
  of the Dart streams.
• BREAKING CHANGE: Removed LoggingEventBus. Reason is that logging can easily
  be implemented with a event listener that listens for all events and logs
  them.