Directly export the Stream constructor.

Also setting up a circular reference back to the
stream as `Stream.Stream`, for backwards-compatibility.

Fixes #1933

lib/stream.js

@@ -26,7 +26,9 @@ function Stream() {
   events.EventEmitter.call(this);
 }
 util.inherits(Stream, events.EventEmitter);
-exports.Stream = Stream;
+module.exports = Stream;
+// Backwards-compat with node 0.4.x
+Stream.Stream = Stream;
 
 Stream.prototype.pipe = function(dest, options) {
   var source = this;