Make ol.source.Source inherit from Observable

The compiler error is:
```
../src/ol/interaction/drawinteraction.js:428: ERROR - Property addFeature never defined on ol.source.Source
    this.layer_.getSource().addFeature(sketchFeature);
```

But why a change from `goog.events.EventTarget` to `ol.Observable` can throw this error?

EDIT: fixed with f68631d61c82e9ec3fcd6be9a7a3b6a2695b8e18