[FIX] Failed URLS will  be added and removed properly.

in SDWebImageManager.m
 a. NSMutableArray *failedURLS => NSMutableDictionary *failedURLS
because I want to using "KEY(NSString)" not object ref for convenient usage.

line 88 :         isFailedUrl = [self.failedURLs containsObject:url]; <= using object ref. not key
This means you should create NSURL once, and reuse it whole lifecycle of application.

Using "Key"(NSString), users do not need to consider about it.
Either using NSURL or NSString will be okay.

 b. Remove failed URL when try again and then success.