Example using an icon symbolizer and displaying a popup

I'm adding this example to demonstrate a few things that should change:

 * the icon symbolizer should accept xOffset and yOffset in pixels
 * the `ol.geom.Geometry.prototype.getCoordinates` method should be exported
 * overlays should have a `panIntoView` method
Export ol.geom.Geometry.prototype.getCoordinates
Add x/y offset for icon symbolizers
Account for icon offsets when doing feature hit detection
Call getUid once
Merge pull request #923 from tschaub/icon

Add xOffset and yOffset support to icon symbolizers.