fix alpha pngs turning into white pngs when caching when you have a method that transforms the image but doesn't touch the pngs
Images can be downloaded in multiple threads.  Therefore the call to self.failedURLs containsObject:url must be synchronized to avoid a collection mutation error.
Merge pull request #388 from cfis/synchronize_failed_urls

Synchronize self.failedURLs
call progress block regardless of completion block, options
Merge pull request #389 from lavoy/master

Calls progress block regardless of completion block and options
Merge pull request #383 from garthex/white_png_issue

fix alpha pngs turning into white pngs
Fix some document error.
Fix a mismatch description.
Adjust indenting.
Format code in document.
Merge pull request #400 from BB9z/fix/document-parameter

Some document improves.
Update SDWebImageCompat.h

Undefine macros before define to prevent warning on iOS
Merge pull request #401 from docallag/master

Prevent warnings on iOS
Fix typo in download page URL
#406 - Fixed a problem where hitting a failed URL more than once wouldn't call the completion block
Merge pull request #408 from dlbuckley/issue-406-Failed-URL-Problem

#406 - Fixed failed URL problem
Custom image cache search paths

This is particularly useful if you are bundling images with your app that have been cached by SDWebImage. (ie. if you are 'seeding' your app with a core-data file that contains a lot of URL's to images and would like to also seed those images without having to copy every one of them over)

For example, you can tell SDImageCache to add '[[NSBundle mainBundle] resourcePath]' as a custom path, so that the main bundle will be queried for cached images.

This prevents the need for you to copy pre-cached images over to the caches/ImageCache folder that SDImageCache normally checks for.

The custom paths are read-only.
Refactor GIF support

Remove GIF specific code from main source, isolate it in a UIImage category to ease future addition of format support
Add WebP format support (fix #410)

How awesome is that?! =)
Added libwebp dependency. Also fix a typo in description.