Export the type checking functions from `util.js`.

As per discussion at:
http://groups.google.com/group/nodejs-dev/browse_thread/thread/b08970166e4670cf

lib/util.js

@@ -383,24 +383,28 @@ function isArray(ar) {
          Array.isArray(ar) ||
          (ar && ar !== Object.prototype && isArray(ar.__proto__));
 }
+exports.isArray = isArray;
 
 
 function isRegExp(re) {
   return re instanceof RegExp ||
       (typeof re === 'object' && objectToString(re) === '[object RegExp]');
 }
+exports.isRegExp = isRegExp;
 
 
 function isDate(d) {
   return d instanceof Date ||
       (typeof d === 'object' && objectToString(d) === '[object Date]');
 }
+exports.isDate = isDate;
 
 
 function isError(e) {
   return e instanceof Error ||
       (typeof e === 'object' && objectToString(e) === '[object Error]');
 }
+exports.isError = isError;
 
 
 function objectToString(o) {