Merge pull request #297 from inamiy/disk-cache

Add synchronous disk-cache loading method.
[BUG] Fix invalid alpha on JPEG files
Synchronize access to SDWebImageManager's mutable structures (fix #301)
Merge pull request #299 from Reflejo/master

Fix "Alpha on JPEG with no alpha" when decoding
Add ability to set custom downloader HTTP headers (fix #171)
Add `SDWebImageManager` delegate allowing fine control of manager's cache-in

- `imageManager:shouldDownloadImageForURL:` let delegate to conditionaly block cache-in (fix #134)
- `imageManager:transformDownloadedImage:` let delegate to transform the image prior to cache-in (fix #63, fix #284)
Add completion block support to `SDWebImagePrefetcher` (fix #127)
Add (kinda) LIFO queue mode support for image downloading (fix #294)
correct cachePolicy constants.

NSMutableURLRequest accepts NSURLRequestCachePolicy as cachePolicy,
not NSURLCacheStoragePolicy.
Merge pull request #302 from n-miyo/fix_cache_policy

Wrong cache policy value.
Prefetching file properties in the disk cleaning enumerator
Merge pull request #307 from nebillo/master

Optimizing enumeration of files while cleaning the disk cache
Detect parent operation cancellation in download operation completion block
Merge pull request #311 from sebreh/master

Detect cancelled parent operation in download operation callback
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