QuickIcon

Quick Icon is an open project that is a tool for  developers that helps them generate necessary icon assets for Xcode.

The App is available on the App Store for free: https://itunes.apple.com/lb/app/quick-icon/id1227275147?mt=12

WARNING: iMessage assets aren't cropped correctly yet. In case you can fix it please do, as I can't find more time to work on it now.