Updated libwebp reference to 0.4.1 (this fixed the arm64 build issues)
Fix misnamed parameters in comment documentation
Merge pull request #850 from tonyarnold/fix-comment-documentation

Fix misnamed parameters in comment documentation
Updated references to libwebp files after the changes from their latest release (0.4.1)
More explicit error message in case of nil completedBlock

NSParameterAssert doesn't provide enough information for understanding the API misuse.
Pointing the API user to SDWebImagePrefetcher makes more sense, as a nil completedBlock most likely signifies the intention of prefetching the image.
Merge pull request #854 from Gabro/patch-1

More explicit error message in case of nil completedBlock
Updated README.md with CocoaDocs link
Merge branch 'master' of https://github.com/rs/SDWebImage