Fixes galore!

• [new] Allow use of NSURLCache via a custom NSURLSession #477 wiseoldduck
• [new] Respect Cache-Control and Expires headers if the cache supports TTL. #462 wiseoldduck
• [new] Updated to latest PINCache beta 7. #461 wiseoldduck
• [iOS11] Fix warnings #428 Eke
• [new / beta] Native Support for GIFs and animated WebP #453 garrettmoon
• [new] Add support for getting NSURLSessionMetrics back. #456 garrettmoon
• [removed] Removed support for FLAnimatedImage #453 garrettmoon
• [new] Add support for higher frame rate devices to animated images. #417 garrettmoon
• [fixed] Fixes non-animated GIFs being delivered as an animated image. #434 garrettmoon
• [fixed] Fixes a bug where using PINRemoteImageBasicCache would cause images to be decoded on the main thread. #457 kgaidis
• [cleanup] Remove unused code that supported iOS < 7. #435 Adlai-Holler
• [cleanup] Use NS_ERROR_ENUM to improve Swift import. #440 Adlai-Holler
• [fixed] Fixes nil session manager configuration. #460 garrettmoon
• [fixed] Fixes deprecated -defaultImageCache not being called if overridden. 479 nguyenhuy
• [new] Add a new API that allows a priority to be set when a new download task is scheduled. #490 nguyenhuy