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.
The callbacksForURL: method of SDWebImageDownloader now returns a copied array. This should fix the crash in multithread environment such as:
Collection <__NSArrayM: 0x226d9310> was mutated while being enumerated.
Merge pull request #471 from PinchProject/hotfix/fast_enumeration_fix

The callbacksForURL: method in SDWebImageDownloader should return a copy of the mutable array!