title: File download with Alamofire
body: <p>I'm downloading a file to a custom file path like this:</p>
  
  <pre><code>let tempFilePath = self.makeTempFilePath()
  Alamofire.download(.GET, downloadURL) { temporaryURL, response in
      return tempFilePath
  }
  </code></pre>
  
  <p>How do I know when the download is complete and the success/failure status? I tried <code>responseData</code> closure but <code>result</code> parameter always appears as "failure".</p>