Add an offset property to ol.Overlay

Replaces "offsetX" and "offsetY" by an ol.Object "offset" property.

As discussed on the [mailing list](https://groups.google.com/d/msg/ol3-dev/dMzTK0o5j8Y/khYCZHP9gpMJ).
We could change this to be consistent with `ol.layer.Base#getOpacity` (for example): the function returns the default value.