Update README with 'info' and 'list' commands.
Add build status to README (using drone.io).
Use Java logging for diagnostic info.

I have incorporated Groovy's @Log annotation to provide loggers for the main
classes. A logger is also available to post-installation scripts via the log
property as well.

I have also switched back to using println() for the normal output of commands.
I originally used an explicit System.out.println() call and put that inside
another method to avoid Groovy initialisation cost. But that cost seems to be
incurred no matter what I do (unless I write Java code), so this 'optimisation'
just isn't worth it.