Added -khtml-user-drag and -webkit-user-drag styles to DnD article for old WebKit support

Safari 4 and earlier don't support the `draggable` attribute, but `-khtml-user-drag: element` / `-webkit-user-drag: element` have the same effect.

This isn't just necromancy; `Modernizr.draganddrop` returns `true` for these browsers, so with the article recommending this as the method for feature detection, without these styles a developer could leave users with a broken interface.

(Any other changes in the diff are because of trailing whitespace trimming by my text editor)