logging mechanism improvements

[jjohannes]

Use a logging mechanism with log levels and reduce default output (It's a lot at the moment where important information gets lost). Remove all System.out.println() statements and replace them with log commands.

E.g.: These are the options of gradle and we could do something similar

• Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.