Restore artificial delay in Multiplex demo.

The change to ASBasicImageDownloader regressed the artificial network
delay in the original demo code.  Revert it.

examples/Multiplex/Sample/ViewController.m

@@ -14,7 +14,7 @@
 #import <AsyncDisplayKit/AsyncDisplayKit.h>
 
 
-@interface ViewController () <ASMultiplexImageNodeDataSource, ASMultiplexImageNodeDelegate>
+@interface ViewController () <ASMultiplexImageNodeDataSource, ASMultiplexImageNodeDelegate, ASImageDownloaderProtocol>
 {
   ASMultiplexImageNode *_imageNode;
 
@@ -33,7 +33,8 @@
 
 
   // multiplex image node!
-  _imageNode = [[ASMultiplexImageNode alloc] initWithCache:nil downloader:[[ASBasicImageDownloader alloc] init]];
+  // NB:  we're using a custom downloader with an artificial delay for this demo, but ASBasicImageDownloader works too!
+  _imageNode = [[ASMultiplexImageNode alloc] initWithCache:nil downloader:self];
   _imageNode.dataSource = self;
   _imageNode.delegate = self;
 
@@ -132,4 +133,50 @@
   }
 }
 
+
+#pragma mark -
+#pragma mark ASImageDownloaderProtocol.
+
+- (id)downloadImageWithURL:(NSURL *)URL
+             callbackQueue:(dispatch_queue_t)callbackQueue
+     downloadProgressBlock:(void (^)(CGFloat progress))downloadProgressBlock
+                completion:(void (^)(CGImageRef image, NSError *error))completion
+{
+  // if no callback queue is supplied, run on the main thread
+  if (callbackQueue == nil) {
+    callbackQueue = dispatch_get_main_queue();
+  }
+
+  // call completion blocks
+  void (^handler)(NSURLResponse *, NSData *, NSError *) = ^(NSURLResponse *response, NSData *data, NSError *connectionError) {
+    // add an artificial delay
+    usleep(1.0 * USEC_PER_SEC);
+
+    // ASMultiplexImageNode callbacks
+    dispatch_async(callbackQueue, ^{
+      if (downloadProgressBlock) {
+        downloadProgressBlock(1.0f);
+      }
+
+      if (completion) {
+        completion([[UIImage imageWithData:data] CGImage], connectionError);
+      }
+    });
+  };
+
+  // let NSURLConnection do the heavy lifting
+  NSURLRequest *request = [NSURLRequest requestWithURL:URL];
+  [NSURLConnection sendAsynchronousRequest:request
+                                     queue:[[NSOperationQueue alloc] init]
+                         completionHandler:handler];
+
+  // return nil, don't support cancellation
+  return nil;
+}
+
+- (void)cancelImageDownloadForIdentifier:(id)downloadIdentifier
+{
+  // no-op, don't support cancellation
+}
+
 @end