make LoggingEventListener extensible

[dbyron-sf]

Would you consider making LoggingEventListener extensible? I think this means making it an open class and making logWithTime protected.

My goal is to tweak the log levels a bit, so things like connectFailed/responseFailed log at warn, but the others log at some level that's determined by a config flag in my app -- either debug, or info when more verbose logging is enabled.

As things are now, it looks like I need to re-implement LoggingEventListener, even though it's so close to what I'm looking for.