Backward compatibility with Jenkins plugin

Since logs migration to java-util-logging, all messages are now on standard error output stream.
Indeed, ConsoleHandler implemented in JDK writes all messages on System.err.

Before this migration, Fitnesse logs was on System.out (and sometimes on System.err). 
Jenkins plugin (https://github.com/jenkinsci/fitnesse-plugin) checks the standard output to know if the server is launched... I will do a pull request soon to change that, but I think that it is a good thing to have log messages in standard output.
So, I have created a console handler, it logs in standard output messages which have a level less than WARNING and in standard error output otherwise.