Issue 711: MyBatis Generator Maven Plugin should log to Maven log

Fixes logging when log4j is not used.  The following error messages will
no longer seen:
log4j:WARN No appenders could be found for logger
(org.mybatis.generator.internal.db.DatabaseIntrospector).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for
more info.