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.
Fix condition logic issue in #326
Fix NSURLResponse not cached when `SDWebImageDownloaderEnableNSURLCache` is passed #326
Force progressive download off when force refresh is on #326
Prevent double disk caching when `SDWebImageRefreshCached` option is used #326
Ensure image isn't decoded twice if not necessary when SDWebImageRefreshCached flag is used #326