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
Typo in README.md
Merge pull request #671 from T-Pham/patch-1

Typo in README.md
Performing cache callbacks from the ioQueue asynchronously.

- more appropriate than dispatch_main_sync_safe, since we’ll always be on the ioQueue when calling
- prevents deadlock situation described in #625
Using the private fileManager instance when on the ioQueue.
Performing getSize and getDiskCount on the ioQueue.
Fixed the background task handled in backgroundCleanDisk.

 - the background task had no effect, since cleanDisk returns immediately and thereby cancels the background task
 - adding cleanDiskWithCompletionBlock: and modifying backgroundCleanDisk to use this method resolves the issue
Exposed cleanDiskWithCompletionBlock:, added some additional documentation and fixed a typo.
Merge pull request #676 from matej/cache-gcd-fixes

Various GCD related fixes for SDImageCache
Bump version to 3.6.0
Fixed issue where cancelled operations aren't removed from runningOperations