Slf4jLogBackend is not installed by default as documentation suggests

[StFS]

I noticed that there was no log output coming from proxy-vole even if I set my logging level to TRACE.

After minor inspection, I saw that the logging backend was null. There is no logging backend set by default.

I would argue that the Logger class should set the SLF4J backend by default. This is actually what the documentation says this library does.

[flyhard]

I think it might be a good idea to remove the Logger class and use Slf4j directly. Anyone who really needs a non-standard logger could just implement a slf4j log forwarder - which would benefit them in any case