Using log4j.xml instead of log4j.properties; closes #70

Also, `log4j.xml` is to be preferred to `log4j.properties`, because the XML file is capable of more advanced configurations.

http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/PropertyConfigurator.html

*"The PropertyConfigurator does not handle the advanced configuration features supported by the DOMConfigurator such as support custom ErrorHandlers, nested appenders such as the AsyncAppender, etc."*