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!
Change version to 3.4
Use 'default_subspec' and separate the WebP support to its sub-spec

This removes unnecessary dependency to 'libwebp' for users who don't need WebP support.
Fix sub-spec dependency
Merge pull request #472 from ikesyo/webp-subspec

Use 'default_subspec' and separate the WebP support to its sub-spec
Update webp to upstream head
Update project to comply with Xcode 5
Fix long running requests
Merge pull request #479 from longlongjump/master

Fix long running requests
Fix xcode project settings
Enforced presence of a completedBlock in downloadWithURL:options:progress:completed:

This method bails out in case of a missing `completedBlock`. While this makes sense (downloading the image without a completion block and not doing anything with it is pointless), a client passing a empty block is not informed about the mistake.

`NSParameterAssert` validates the input and it informs the client about bad usage of the API.