add convienience methods for log4j loggers

I don't want `def logInfo(logger: Logger) { logger.setLevel(Level.INFO) }`, because the method will have to be turned into a function anyway when passed to the `RichLogger` methods, and this way I don't rely on Scala magic to convert the method into a function.

Is there any advantage to having `def logInfo = (l: Logger) => ...` instead of `val logInfo = (l: Logger) => ...`?