Changed icon to match selectmenu widget

http://wiki.jqueryui.com/w/page/12138056/Selectmenu

Probably the markup of the button should changed too to match the button widget from jQuery UI Core - so it could fit nicely into existing projects.

Author: suit

src/jquery.multiselect.js

@@ -55,7 +55,7 @@
       // jQuery UI 1.9+, and otherwise fallback to a custom string.
       this._namespaceID = this.eventNamespace || ('multiselect' + multiselectID);
 
-      var button = (this.button = $('<button type="button"><span class="ui-icon ui-icon-triangle-2-n-s"></span></button>'))
+      var button = (this.button = $('<button type="button"><span class="ui-icon ui-icon-triangle-1-s"></span></button>'))
         .addClass('ui-multiselect ui-widget ui-state-default ui-corner-all')
         .addClass(o.classes)
         .attr({ 'title':el.attr('title'), 'aria-haspopup':true, 'tabIndex':el.attr('tabIndex') })