LOG4NET-342 added the property Configured to ILog that allows to check if the repository was configured properly

Usecase:
   if (log.Configured == false) 
   { 
      throw new ConfigurationErrorsException("Log4net failed to initialize properly."); 
   }

git-svn-id: https://svn.apache.org/repos/asf/logging/log4net/trunk@1489736 13f79535-47bb-0310-9956-ffa450edef68