Fix downloader queue using assign instead of strong under iOS 6
Add synchronous disk-cache loading method.
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.