Add proper logging framework

[kiudee]

Currently we use System.out.println() calls to output general information, warnings and debug code.

In order to offer more control over the output we should switch to a logging framework like Simple Logging Facade for Java (SLF4J).

Using it will be as simple as System.out.println():

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class HelloWorld {
  public static void main(String[] args) {
    Logger logger = LoggerFactory.getLogger(HelloWorld.class);
    logger.info("Hello World");
  }
}