Export ol.source.Source#getExtent
Merge pull request #924 from elemoine/getextent

Export ol.source.Source.prototype.getExtent
Accept null crs
Merge pull request #927 from tschaub/null-crs

Accept null crs in GeoJSON.
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.
Fix the IE pointer event detection

Check 'pointerId' to decide if an event is a pointer events.
Add documentation in mapbrowserevent.js
Change previous_ local variable type to ol.Pixel

And rename it to previousMousePosition_ for clarity