Wraps a Log4j Logger. This non-public class is the one actually interacting
with the log4j.jar library. That way LogWrapper can safely attempt to use
log4j.jar, but still degrade gracefully and provide logging via standard-out
even if log4j is unavailable.
The interactions with log4j.jar could be done directly inside LogWrapper
as long as the Java code is compiled by Java 1.4 or greater (still works
at runtime in Java 1.3). The interactions with log4j.jar only need to be
pushed out into a separate class like this for people using a Java 1.3
compiler, which creates bytecode that is more strict with depedency
checking.