documented workaround for frameworks that extend optional libraries
Added method to SDWebImageManager to check if an image exists in either the disk OR the memory cache
add category for imageView's highlighted state
Performing cache callbacks from the ioQueue asynchronously.

- more appropriate than dispatch_main_sync_safe, since we’ll always be on the ioQueue when calling
- prevents deadlock situation described in #625
Using the private fileManager instance when on the ioQueue.
Performing getSize and getDiskCount on the ioQueue.
Fixed the background task handled in backgroundCleanDisk.

 - the background task had no effect, since cleanDisk returns immediately and thereby cancels the background task
 - adding cleanDiskWithCompletionBlock: and modifying backgroundCleanDisk to use this method resolves the issue
Exposed cleanDiskWithCompletionBlock:, added some additional documentation and fixed a typo.
Merge pull request #676 from matej/cache-gcd-fixes

Various GCD related fixes for SDImageCache
Bump version to 3.6.0
Fixed issue where cancelled operations aren't removed from runningOperations
Updates progress callback to use NSInteger

Pull Request #606 resolves this issue in the code but not in the Documentation.
Merge pull request #685 from jeffleeismyhero/patch-1

Updates progress callback to use NSInteger
Create NSOperation when start operation.
Method getDiskCount returns NSUInteger.
Port Max's crash fix to my 3.6 fork
Merge pull request #694 from shifu/master

Port Max's crash fix to my 3.6 fork
Update SDWebImageDownloaderOperation.m

Fixing race condition that causes cancellation of one download operation to stop a run loop that is now used for another download operation. Race is introduced through -performSelector:onThread:withObject:waitUntilDone:
Merge pull request #698 from salling/master

Fixing race in SDWebImageDownloaderOperation leading to erroneous timeout.
Save image in cache before calling completion block
Merge pull request #700 from herrernst/master

Save image in cache before calling completion block