Better DFImageTaskCompletion completion block

[kean]

Changes to completion block:

• Remove user info dictionary
• Add error argument
• Add DFImageResponse (new class) argument
• Add DFImageTask argument

Completion block should follow simple rules:

• If there is an error, there is no image and no response
• If there is no image and no response, there is always an error

Other changes:

• DFImageFetching protocol and DFImageManagerImageLoader should no longer use DFImageResponse class
• Add public response (DFImageResponse) property to DFImageTask
• Update DFCompositeImageTaskImageHandler to reflect DFImageTaskCompletion changes
• Update DFImageView interface to reflect DFImageTaskCompletion changes

These changes would allow for better compiler checks, better discoverability of APIs and better performance.

[kean]

Done