Definir barra de progresso para baixar o NSData
NSURL *url = [NSURL URLWithString:@"http://i0.kym-cdn.com/entries/icons/original/000/005/545/OpoQQ.jpg?1302279173"];
NSData *data = [NSData dataWithContentsOfURL:url];
imageView.image = [[[UIImage imageWithData:data];
Eu quero definir barra de progresso durante o download.