Replace java.util.logging to slf4j

This modifies fluent-logger to use slf4j http://www.slf4j.org/ instead of java.util.logging. 

With this change, the users can replace logging output to their favorite one, including the previous java.util.logging, Apache's commons-logging, logback, etc. See http://www.slf4j.org/manual.html#swapping for the available loggers
 
In the test code, I added an example of using logback (http://logback.qos.ch/), an implementation of slf4j API.