ClassLauncher: debug when scijava.log.level=debug

We do not replicate the entire SciJava logging system property feature
here, but merely use a quick approximation for the most common case.
We cannot, unfortunately, reuse the LogService infrastructure because
we do not want imagej-launcher to depend on scijava-common at runtime.

For discussion, see:
https://github.com/imagej/imagej-launcher/pull/4#issuecomment-42756021