Nuke 3.1.0

• #64 Fix a performance regression: images are now decoded once per DataTask like they used to
• #65 Fix an issue with custom on-disk cache (ImageDiskCaching) which was called setData(_:response:forTask:) method when the error wasn't nil
• Add notifications for NSURLSessionTask state changes to enable activity indicators (based on kean/Nuke-Alamofire-Plugin#4)