Command-line interface plugin updated for RubyMotion 2.20

This is meant to be applied _after_ RubyMotion 2.20 has been released, which will contain a new cleaned-up CLI based on [CLAide](https://github.com/CocoaPods/CLAide).

* This first commit only really changes the way the plugin is loaded. (Not any longer via `~/Library/RubyMotion`, but instead as part of the gem in `lib/motion_plugin.rb`.)
* The second commit updates the command classes to use the new CLI and gets rid of all deprecation warnings. When and how you want to integrate that is not part of the patch. E.g. you might want to keep the deprecated commands around until a major release.