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