Add offset support to icons

A couple things should change to allow people to use icons as point symbolizers.

 - [x] Export `ol.geom.Geometry.prototype.getCoordinates`.  Currently, we really only make it easy to position overlays at the mouse position (with `overlay.setPosition(evt.getCoordinate())`).  For point symbolizers, you want to position an overlay relative to the geometry coordinates, and these are likely different than the mouse position.  Since the `getCoordinates` method on geometries returns `ol.Coordinate` or arrays of the same, it feels like a natural and safe method to expose.
 - [x] Icon symbolizers should accept `xOffset` and `yOffset` in pixels (so icons don't have to be centered on point geometries).