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).
What about:
```
this.setOffset(goog.isDef(options.offset) ? options.offset : [0, 0]);
```
and remove `ol.Overlay.offset_`, the check in `ol.Overlay.prototype.updatePixelPosition_` and the `undefined` type in `[get|set]Offset` ?