Fix enum plugin to be standard compatible

The word `enum` is is a reserved JavaScript word and form
$.fn.dataTable.enum broke at least YuiCompressor minification

sorting/enum.js

@@ -24,7 +24,9 @@
 var unique = 0;
 var types = $.fn.dataTable.ext.type;
 
-$.fn.dataTable.enum = function ( arr ) {
+// Using form $.fn.dataTable.enum breaks at least YuiCompressor since enum is
+// a reserved word in JavaScript
+$.fn.dataTable['enum'] = function ( arr ) {
 	var name = 'enum-'+(unique++);
 	var lookup = window.Map ? new Map() : {};