Re-render on feature changes

This PR fixes #1798. It should also fix a bug that was reported on the [mailing list](https://groups.google.com/d/msg/ol3-dev/AqbeFEwttuM/ke2ebsX9GzMJ).

The vector source now listens to both "change" and "propertychange" from its features. And with this there's no need to trigger a "change" event when the geometry is changed on the feature – the "propertychange" event is sufficient.