2.4.2 - Optional Welcome

Add

• Accept nil as valid URL parameter for image view's extension methods.

Fix

• The completion handler of image view setting method will not be called any more if self is released.
• Improve empty task so some performance improvment could be achieved.
• Remove SwiftLint since it keeps adding new rules but without a back compatible support. It makes the users confusing when using a different version of SwiftLint.
• Removed Implicit Unwrapping of CacheType that caused crashes if the image is not cached.