Log4J2 fails to recognise the Appender

[brett--anderson]

Log4J2 XML configuration no longer has an 'Appender' sub element to the 'Appenders' group. So the example xml: <Appender name="honeybadger" class="com.workable.honeybadger.log4j.HoneybadgerAppender"> doesn't work. I think the appender needs to be written differently to work with Log4J_2_.

[brett--anderson]

The new Appender is working fine, please let me know if there's anything I can do to improve the code in the pull request.