Accept null crs
Add ol.CollectionEvent#getElement
Use ol.CollectionEvent#getElement in LayerGroup
Export ol.CollectionEvent#getElement
Use ol.Collection#getElement in tests
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.
Move png to data to work with host-examples task
Add documentation in mapbrowserevent.js
Change previous_ local variable type to boolean

And rename it to pressed_ for clarity; the variable represents whether
the mouse button is pressed
Merge pull request #929 from elemoine/collectionevent

Add ol.CollectionEvent#getElement
Remove pressed_ variable and rely on down_ to check if a mouse button is pressed
Fix unclosed div in examples/icon.html
Merge pull request #932 from fredj/unclosed_div

Fix unclosed div in examples/icon.html
ol.layer.Vector pass original options to parent

This makes it possible to pass arbitrary property to ol.layer.Vector and use layer.get('property_name') to get the value of properties.