title: Activity indicator in SdWebImage issue
body: <p>I have a horizontal collection view and it has 20 cells with UIImageView
  I just want to show the activity indicator till the image download completed. I am using the new SDWebImage library where we have methods as <strong>sd_setImageWithURL</strong>
  Till now What i am doing is</p>
  
  <pre><code>__block UIActivityIndicatorView *activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
  activityIndicator.center = cell.photoOneImageView.center;
  activityIndicator.hidesWhenStopped = YES;
  
  [cell.photoOneImageView sd_setImageWithURL:[NSURL URLWithString:@"https://scontent-sjc.xx.fbcdn.net/hphotos-xpa1/v/t1.0-9/p480x480/11073324_10153728863852926_4010319763478440264_n.jpg?oh=590934059508b7da235a46fc39e08063&amp;oe=55B61458"] 
  placeholderImage:[UIImage imageNamed:@"placeholder.jpg"] 
  completed:^(UIImage *image, NSError *error,  SDImageCacheType cacheType, NSURL *imageURL) {
      [activityIndicator stopAnimating];
      [activityIndicator removeFromSuperview];
      }];
  
  [cell.photoOneImageView addSubview:activityIndicator];
  [activityIndicator startAnimating];
  </code></pre>
  
  <p>I am writing this code in <strong>cellForItemAtIndexPath</strong> method
  It shows sometimes multiple indicators on 1 cell and they are not removing also sometimes when we scroll the collection view horizontally.</p>
  
  <p>I saw this <a href="https://github.com/rs/SDNetworkActivityIndicator" rel="nofollow">https://github.com/rs/SDNetworkActivityIndicator</a> but I am unable to use this. No luck with that. Has anyone implemented activity indicator in tebleview cell or collection view cell when the image completed the indicator vanish. Please help. Any help will be appreciated. Thanks</p>