Failed-URL-Retries-in-SDWebImageManager

Using the SDWebImageView category I sometimes run into the problem that a request failed, although the image link is NOT broken. 

Other people seem to be experiencing the same problem: https://github.com/rs/SDWebImage/issues/363

These small changes allow to set a variable on the image manager, which defines a number of allowed retries before the image link is marked as broken by adding it to the failedURLs-array.