Added a new constructor to SDWebImagePrefetcher

that allows it to be instantiated with any image manager

@@ -71,6 +71,11 @@ typedef void(^SDWebImagePrefetcherCompletionBlock)(NSUInteger noOfFinishedUrls,
 + (SDWebImagePrefetcher *)sharedImagePrefetcher;
 
 /**
+ * Allows you to instantiate a prefetcher with any arbitrary image manager.
+ */
+- (id)initWithImageManager:(SDWebImageManager *)manager;
+
+/**
  * Assign list of URLs to let SDWebImagePrefetcher to queue the prefetching,
  * currently one image is downloaded at a time,
  * and skips images for failed downloads and proceed to the next image in the list

@@ -37,8 +37,12 @@
 }
 
 - (id)init {
+    return [self initWithImageManager:[SDWebImageManager new]];
+}
+
+- (id)initWithImageManager:(SDWebImageManager *)manager {
     if ((self = [super init])) {
-        _manager = [SDWebImageManager new];
+        _manager = manager;
         _options = SDWebImageLowPriority;
         _prefetcherQueue = dispatch_get_main_queue();
         self.maxConcurrentDownloads = 3;