Use slf4j to log errors and information not System.out / System.error

[e-roeling]

We use elastic search to collect the logging of all environments. For environments on which FitNesse is running we run into issues with stack traces, this is caused by the fact that they are just printed to System.out.

I see that some classes already make use of slf4j. It would help us if this can be done for all classes (Except for JUnitTeamcityReporter as the logging in this class if from a different nature.)

I will add a pull request for this.