Update SDWebImagePrefetecher.m

I added progress block that I need.
Merge pull request #634 from burakkilic/patch-1

Update SDWebImagePrefetecher.m
Update SDWebImagePrefetecher.m

I added "+ self.skippedCount" because it never completes if there are skipped operations.
Merge pull request #635 from burakkilic/patch-1

Update SDWebImagePrefetecher.m
Missing method definition in header file
Merge pull request #637 from runmad/master

Missing method definition in header file
added clearDiskOnCompletion
Merge pull request #638 from limbo-lab/master

added clearDiskOnCompletion
Bump version to 3.5.4
Fix deployement target for arm64
add category for imageView's highlighted state
Check progressBlock is not nil before executing.
Merge pull request #647 from musaicapp/bug

Prefetcher: check progressBlock is not nil before executing.
Add typedef for cache operation complete block

Makes it cleaner when users want to define the block independantly
Merge pull request #649 from finik/patch-1

Add typedef for cache operation complete block
add support for high priority download
Merge pull request #664 from xuki/master

add support for high priority download
Fix some indentation issues
SDWebImageDownloaderOperation: call endBackgroundTask after operation is finished so that beginBackgroundTaskWithExpirationHandler and endBackgroundTask are balanced
Merge pull request #665 from taichino/fix_backgrounding_task

Call endBackgroundTask at the end of start method