New option SDWebImageRefreshCached

Even if the image is cached, fetch the URL again anyway. When set,
NSURLCache is enabled in the downloader via the new option
SDWebImageDownloaderEnableNSURLCache.

NSURLCache will handle the protocol caching while SDWebImage remains
useful for offline images.

This option helps deal with images changing behind the same request URL,
e.g. Facebook graph api profile pics where the request URL
https://graph.facebook.com/[userid]/picture returns a redirect to the
actual profile image.

If a cached image exists, the completion block is called once with the
cached image and again with the final image.