fixes lou#49: new option to select nested optgroup options

Check if attribute is a DOM node before copying it. Plugin was not wo…

…rking on IE7.

fix mouse hover

authorize option with blank value to be selected

fix scroll

Merge branch 'keyboard'

Conflicts:
	css/multi-select.css
	js/application.js

fix lou#16: class was set to undefined when no class was given

Merge pull request lou#14 from lou/callbackOnInit

Callback on init