Events handling

I implemented some basic event handling, but i'm not sure, if this is what we were talking about. 
Can you please look at it?

Basic API is shown in tests. Example:

```dart
mountComponent(span({
  "id": "wrapper",
  "onClick": (Component component, Event event) {
     /* do something */
  }
}, [
  span({
    "onClick": (Component component, Event event) {
     /* do something else */
    }
  })
]), mountRoot);
```

I created type EventListener to easy check type of object passed to onEvent attribute.

[drone.io](https://drone.io/github.com/cleandart/tiles/34)