title: SDWebImage not caching to disk
body: <p>I'm trying to use SDWebImage to cache images retrieved from the web. 
  The image gets cached to memory all the time. I've seen the documentation that saving to disk is default. </p>
  
  <blockquote>
    <p>Everything will be handled for you, from async downloads to caching management.</p>
    
    <p>By default SDImageCache will lookup the disk cache if an image can't be found in the memory cache. You can prevent this from happening by calling the alternative method imageFromMemoryCacheForKey:.</p>
  </blockquote>
  
  <p>However every time I relaunch my application, SDWebImage downloads the file again from the web and not retrieving it from the disk.
  I will receive <strong>SDImageCacheType</strong> of value <strong>SDImageCacheTypeNone</strong> when my application reloads, and <strong>SDImageCacheTypeMemory</strong> when I keep scrolling without exiting my application. </p>
  
  <p>Any ideas? I've tried caching manually but the image always appear black. Not sure what where wrong. Any help is appreciated!</p>
  
  <p>I'm using this method to download the images:</p>
  
  <pre><code>- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
  </code></pre>