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).
I'm happy to change the way we do it for "positioning" if you think it makes more sense.