separate core functionality from default logging

[vorburger]

For a new personal project I'm toying with, I'd actually prefer it now if this great old library of mine would separate its core functionality from the currently built-in and on by default logging it does.

If I find the time for it, I therefore may split exec into exec-core (which would have no dependency on slf4j, or any other logging framework) and exec-slf4j some day.

[vorburger]

I accept SLF4j as the de-facto Java logging library, and no longer need this.

But #116 is a little bit related to this (it's a more narrow special case), and is something I do want.