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.
Overload -setImageWithURL to delay setting of placeholder image until after loading
Moved the delay placeholder flag to SDWebImageOptions
Updated bitwise operator to correctly find option if more than 1 are included
Wrapped placeholder check in parens to work with bitwise
Adds -[SDWebImageManager saveImageToCache:forURL:] method
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.
Documented SDWebImageDownloaderOperation initWithRequest:options:progress:completed:cancelled: method. Fixes #526
Merge pull request #701 from billburgess/delayplaceholder

Delay the loading of the placeholder image until after load
Keep a reference to the image URL
Add documentation to imageURL property
Add imageURL property to remaining categories
Scratch that, this won't work on UIButton
Store image URLs by state in the UIButton category
Conform to coding style