gistfile1.js

(function($) {
        $.fn.limit = function( n, callback ) {
                callback = typeof( callback ) === 'function' ? callback : $.noop;
                var elements = this;
                return elements.click( function(e) {
                    var checkedCount = elements.filter(':checked').length;
                    if (checkedCount > n && !callback(checkedCount)) {
                        e.preventDefault();
                    }
                });
        };
})(jQuery);