stop printing to sdtout/stderr

[shalom938]

we use posthog java in an intellij plugin, some users have connection issues because of firewalls/company proxies/ ssl certificates etc.
errors are printed to stdout/err and Idea log get exploded, HttpSender does e.printStackTrace()
its probably better to use some logging abstraction like JUL so logging level can be controlled.

[martypitt]

Came here to find this.

Please add SLF4J as a logging facade, so people can wire in their own loggers.

I'd be happy to put this on a PR - but it looks like PR's aren't getting merged...