2.20.0

• Removed the delegate method onInAppMessageHTMLButtonClicked:buttonID: from ABKInAppMessageControllerDelegate protocol.
• Adds Carthage support.
• Fixes a multithreading issue where logging custom events from different threads would sporadically cause errors.
• Now the onInAppMessageHTMLButtonClicked:clickedURL:buttonID: delegate method will be called every time a URL is clicked. The method used to be only called when there was a button ID in the URL link.
• Fixes the issue where a close button's color on modal and full in-app messages didn't respect the opacity value.
• Updates the feedback element to reject messages that contain only whitespace.
• Updates remote push handling to call the completion handler passed in every time (a code path previously existed that would return without calling it).
• Fixes an issue where failure to download HTML in-app message assets mid-download resulted in display without assets.