Add -calculateSizeWithCompletionBlock:.

This method provides a way to asynchronously calculate the size of the
disk cache, reporting both the number of files and the total file
size.

This is useful when querying large disk caches because the file system
enumeration operation can take some time.
Merge pull request #434 from jparise/calculate-size

Add -calculateSizeWithCompletionBlock:.
Remove an unused call to -callbacksForURL:.

We don't currently signal cancellation via a callback.
Merge pull request #436 from jparise/unused-callbacksForURL

Remove an unused call to -callbacksForURL:.
Break a retain cycle
Merge pull request #444 from hsoi/scrolling-lockup

Break a retain cycle
Update README.md

Minor fix to callback arguments. NSData included now.
Merge pull request #446 from tissak/master

Tiny Doc fix
Use an empty string to if str was NULL.
added multiple download of images for animationImages property of UIImageView
Added background clean disk to UIApplicationDidEnterBackgroundNotification #306
Merge branch 'master' of git://github.com/gringoireDM/SDWebImage into gringoireDM-master
Fix downloadImageWithURL:options:progress:completed returning nil (fix #464)
Call handlers on the main thread synchronously to enhance responsivity (fix #462)
Always call the completedBlock from main thread (fix #397)

This is not a good practice but doing otherwise leads to a lot of mistakes
Try to fix the thread block (#399)
Fix unreleased operation (fix #466, fix #444)
Fix the dead thread bug on iOS 5 (fix #466)
Update README.md
Canceling the SDWebImageOperation will also cancel the expensive disk cache query operation.