Replace System.{err,out}.println with logging

[morungos]

We use log4j, and I can't see any good reason for directly writing to the console. It's way better to use a logger. Even if we want to write to the console, we can make an appender to do that, but at least we can factor that and manage it.

[morungos]

Actually, we use slf4j too!! We should probably choose one of them alone and use that instead. I'd pick slf4j.