Use SLF4J instead of Log4J

This would allow programs that use the jediterm library to use the logger of their choice instead of requiring that they use log4j since slf4j is a logging facade that supports multiple loggers.

I included the slf4j-simple logger so that when the jediterm sample programs can still produce INFO-level logging.

(In a couple of cases my IDE removed unused imports.)

This is dependent upon a pty4j built with slf4j instead of log4j (see traff/pty4j#2).