Node placeholder API

ASDisplayNodes now have an overidable method -placeholderImage that lets you provide a custom UIImage to display while a node is displaying asyncronously. The default implementation of this method returns nil and thus does nothing. A provided example project also demonstrates using the placeholder API.

AsyncDisplayKit/ASDisplayNodeExtras.mm

@@ -122,6 +122,15 @@ extern id ASDisplayNodeFindFirstSubnodeOfClass(ASDisplayNode *start, Class c)
   });
 }
 
+#pragma mark - Placeholders
+
+UIColor *ASDisplayNodeDefaultPlaceholderColor()
+{
+  return [UIColor colorWithWhite:0.95 alpha:1.0];
+}
+
+#pragma mark - Hierarchy Notifications
+
 void ASDisplayNodeDisableHierarchyNotifications(ASDisplayNode *node)
 {
   [node __incrementVisibilityNotificationsDisabled];