Replaced log4j dependency with SLF4J

I don't use log4j, so I needed to replace the logging dependency with something that would work for me.  SLF4J is a great library for creating a nice API that can be used as a facade for a multitude of logging frameworks.  It's highly recommended as a dependency replacement for log4j.

Great work on the Expect implementation, btw!  I looked at the other Expect Java libraries before coming here, and neither of them worked the way that I wanted.  Your implementation is much simpler, making it easier to modify and integrate into other projects.