Merge pull request #219 from SRandazzo/patch-1

Fix documentation directive
Fix CGBitmapContextCreate errors (fix #204)
Add activity indicator demo
Fix a race condition (fix #220)
Improved background image decoding performance.

Tests on large images indicate an up to 4x improvement with regard to the time spent in decodedImageWithImage:.
Disk cache now uses the raw image data returned from the server.

This preserves the image quality and retains some image attributes, such as the alpha channel (for non-jpeg images).
Passing the exact cache type in the completion block.

- none, disk and memory
- can still be used as a bool (if true there was a cache hit)
Implemented progress callbacks and related fixes.
Always making callbacks using the main thread.

Even if using the downloader directly without SDWebImageManager.
Calling completion callbacks in the main thread.

- made sure this is true even if the downloader is used on its own
- SDWebImageManager doesn't need to perform a dispatch to the main thread any longer
Resolved some downloader threading issues.

- callbacks could be released between the callback existence if check and invocation
- could be nilled out in a background thread call to done, while being prepared for invocation on the main thread
- now making sure done is always performed on the main thread
- also added some related threading fixes and optimizations
Making sure cache queries always callback in the main thread.

- queryDiskCacheForKey:done: now always invokes the callback in the main thread
- removed the now redundant main thread dispatch in the image manager class
Fix demo app regarding last changes
Try to fix decode image error message
Do not init an activity indictor if image isn't downloading in demo app (good practice)
Fix bug in SDWebImagePrefetcher where startPrefetchingAtIndex would never be called due to is always being 0
Fix SDWebImagePrefetcher maxConcurrentDownloads property
Do not capture self in operations in categories
Try to fix a random crash on dealloc
Remove no longer necessary cleanMemory on app going to background as NSCache does this by itself