documented workaround for frameworks that extend optional libraries
Added method to SDWebImageManager to check if an image exists in either the disk OR the memory cache
Fixed issue where cancelled operations aren't removed from runningOperations
Create NSOperation when start operation.
Method getDiskCount returns NSUInteger.
fix UIImageView+HighlightedWebCache call error method
Resolve race condition.

While exceedingly unlikely, the old construct introduced potential race condition where it checked wself first, and assigned sself second. This now assigns sself first, and then checks that, which is the correct pattern, already used elsewhere in SDWebImage.
Adjusted completion logic in prefetcher, as the old implementation would finish prematurely if any requests were skipped.

Also adjusted documentation in headers to make it clear that the `finishedCount` includes both successful and unsuccessful requests.
Exposed SDWebImageManager cacheKeyForURL
Merge pull request #644 from kencoken/master

Added method to SDWebImageManager to check if an image exists in either ...
Exposed cache paths - fixes #339
Merge pull request #687 from rokemoon/master

Create NSOperation when start operation.
#761 fixing potential bug when sending a nil url for UIButton+WebCache
Merge pull request #763 from dchohfi/master

#761 fixing potential bug when sending a nil url for UIButton+WebCache
Merge pull request #376 from ivanoblomov/master

Added alternate compiler flag for frameworks that extend optional libraries, like Parse and RestKit
Updated README.md
SDWebImageManager uses the shared instance of SDWebImageDownloader. This was requested by several developers for easier setup of their downloads. Replaces #486
Merge pull request #683 from brynbodayle/master

Fixed issue where cancelled operations aren't removed from runningOperations.
Exposing the SDWebImageManager from the SDWebImagePrefetcher class. Fixes #394
Merge pull request #765 from Whirlwind/fix_highlight_error

fix UIImageView+HighlightedWebCache call error method
should not add url to failedURLs when timeout, cancel and so on. #707