Support function input to selectClass option.

So you can pass a function into the selectClass option. Pass $el as 'this' to the function to allow context-aware class definitions.

For example:

  $('select').uniform({
    selectClass: function() {
      var cls = this.data('uniform-class');
      return (cls !== undefined && cls.length > 0 ? 'selector ' + cls : 'selector');
    }
  });

jquery.uniform.js

@@ -782,7 +782,7 @@ Enjoy!
 					}
 
 					ds = divSpan($el, options, {
-						divClass: options.selectClass,
+						divClass: (typeof options.selectClass === "function" ? options.selectClass.apply($el) : options.selectClass),
 						spanHtml: ($el.find(":selected:first") || $el.find("option:first")).html(),
 						spanWrap: "before"
 					});