KnowledgeRuntimeLogger should be activable by enabling category logging and send it's output to slf4j

Drools's audit logging should log to slf4j, so the user can easily configure advanced logging features such as rolling files, zipped files, ...

Also enabling the logging should be user friendly. So instead of this:

KnowledgeRuntimeLogger logger =
  KnowledgeRuntimeLoggerFactory.newFileLogger(ksession, "logdir/mylogfile");
...
logger.close();

They just enable it in the logger configuration (logback.xml or log4j.xml):

  <logger name="org.drools.logger.audit" level="trace"/>

This way, drools's audit logging is no longer the odd duck in the normal logging output.