                                                 request finishes. Defaults to `nil`.
         - parameter filter:                     The image filter applied to the image after the image request is
                                                 finished. Defaults to `nil`.
+        - parameter progress:                   The closure to be executed periodically during the lifecycle of the
+                                                request. Defaults to `nil`.
+        - parameter progressQueue:              The dispatch queue to call the progress closure on. Defaults to the
+                                                main queue.
         - parameter imageTransition:            The image transition animation applied to the image when set.
                                                 Defaults to `.None`.
         - parameter runImageTransitionIfCached: Whether to run the image transition if the image is cached. Defaults

                                                request finishes. Defaults to `nil`.
        - parameter filter:                     The image filter applied to the image after the image request is
                                                finished. Defaults to `nil`.
        - parameter imageTransition:            The image transition animation applied to the image when set. 
                                                Defaults to `.None`.
        - parameter runImageTransitionIfCached: Whether to run the image transition if the image is cached. Defaults
                                                to `false`.

        URLRequest: URLRequestConvertible,
        placeholderImage: UIImage? = nil,
        filter: ImageFilter? = nil,
        progress: ImageDownloader.ProgressHandler? = nil,
        progressQueue: dispatch_queue_t = dispatch_get_main_queue(),
        imageTransition: ImageTransition = .None,
        runImageTransitionIfCached: Bool = false,
        completion: (Response<UIImage, NSError> -> Void)? = nil)
