Add support for using Log4j as a global Tomcat library.

I would like to be able to use log4j-api and log4j-core in my global Tomcat lib directory and have it both integrate with JULI (which the log4j-jul module may help, though there could be more work involved) as well as properly integrate with any shutdown logic Tomcat provides to delay shutdown of the logging system until the very end.

If possible, it would be great if this could either be generic enough or extensible enough to allow the same for Jetty, too.