2.0.0 - Kingfisher 2

Add

• OS X support. Now Kingfisher can work seamlessly for NSImage. #201
• watchOS 2.x support. #210
• Swift Package Manager support. #218
• Unified KingfisherOptionsInfo API. Now all options across the framework are represented by KingfisherOptionsInfo with type same behavior. #194
• API for changing download priority of image download task after the download started. #73
• You can cancel image or background image downloading task now for button as well. #205

Fix

• A potential thread issue when asking for cache state right after downloading finished.
• Improve MD5 calculating speed. #220
• The scale was not correct when processing GIF files.