Animated image category, bug fixes
- Add -calculateSizeWithCompletionBlock
- Add multiple download of images for animationImages property of UIImageView
- Add background task for disk cleanup #306
- Fix dead thread issue on iOS 5 #444, #399, #466
- Make IO operations cancelable to fix perf issue with heavy images #462
- Fix crash "Collection <__NSArrayM: ...> was mutated while being enumerated." #471