SDWebImagePrefetcher should only perform `NSLog` statements if `SD_VERBOSE` is defined

Right now, `SDWebImagePrefetcher` is very verbose. I'd suggest removing the `NSLog` statements altogether, or perhaps use some compile-time constant (like you did for `SD_WEBP`) like I did in this pull request. I changed this so  the `NSLog` statements are only invoked if `SD_VERBOSE` is defined. Otherwise, the `NSLog` statements are omitted.