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 did not do that because we don't do something similar for "positioning". If you don't set any "positioning" then getPositioning returns undefined although a default positioning value will be used (top-left) in updatePixelPosition_. For consistency reasons I've wanted "offset" to work the same way. What do you think? Would you change the way "positioning" works?