Modified the cost calculuations to appropriately compute the 'scale' of an image
fix #610
Adds setter to override default SDWebImageDownloaderOperation.
atomicity was implicit (compiler complained). Declared 'nonatomic' as that's fairly standard on iOS
Switch to built in XCTest async
Handle empty urls NSArray

In some cases the user may end up with an empty urls NSArray, in which case the completion block is never called.
This commit handle such case to call immediately the completion block (if any)
Merge pull request #929 from kirualex/patch-2

Handle empty urls NSArray
Decoding webp, depends on source image data alpha status.
Implement 'SDWebImageDownloaderAllowInvalidSSLCertificates' option.
Fixed #774 - remove deprecated methods. Since 4.0 is a major release, we don't need the backwards compatible methods
Merge pull request #937 from pavelosipov/master

Implement 'SDWebImageDownloaderAllowInvalidSSLCertificates' option.
Merge pull request #936 from flexih/master

Decoding webp, depends on source image data alpha status.
Merge pull request #898 from mattjgalloway/xctest_async

Switch to built in XCTest async
Merge pull request #886 from hsoi/fix-implicit-atomicity-declaration

Fix implicit atomicity declaration of the SDWebImageManager.cacheKeyFilter
Merge pull request #876 from matias-pequeno/custom_operations

Allows user to override default SDWebImageDownloaderOperation
Merge pull request #840 from foxling/master

fix #610 display progressive jpeg issue
Merge commit '7d9337406760603ddee47d5515e1e4835618b6b2' into cost_compute_fix
304 http status code handling
Increased default maxConcurrentOperationCount, fixing issue #527
Merge pull request #897 from MrAlek/fix/max-concurrency-count

Increased default maxConcurrentOperationCount, fixes #527