Allow relative URLs for 'icon' property

[ccnokes]

The icon option in HTML5 Notifications allows relative URLs without a problem. It looks like NSURL or something wants it to be absolute with a scheme.

Using a relative URL will throw this warning: CFURLCopyResourcePropertyForKey failed because it was passed an URL which has no scheme and the icon won't show up. I thought I would just mention that, I don't know ObjC so maybe there's a fix for it or maybe it would just be a documentation change.