Fix retain cycle in basic, cache-less ASNetworkImageNodes

This should not directly affect more complex apps that don't use the ASBasicImageDownloader.

Also disables the default-on text placeholders, as they churn memory during the measurement pass.
These were intended to be written with pure layers (without using backing stores), so I don't
think it is a reasonable default-on behavior until that is fixed.

examples/Kittens/Sample/ViewController.m

@@ -166,8 +166,6 @@ static const NSInteger kMaxLitterSize = 100;        // max number of kitten cell
 
 - (void)tableView:(UITableView *)tableView willBeginBatchFetchWithContext:(ASBatchContext *)context
 {
-  NSLog(@"adding kitties");
-    
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
     sleep(1);
     dispatch_async(dispatch_get_main_queue(), ^{
@@ -189,8 +187,6 @@ static const NSInteger kMaxLitterSize = 100;        // max number of kitten cell
       [tableView insertRowsAtIndexPaths:indexPaths withRowAnimation:UITableViewRowAnimationFade];
 
       [context completeBatchFetching:YES];
-
-      NSLog(@"kittens added");
     });
   });
 }