Use gradle-one-jar plugin; create main() skeleton

Unfortunately, it appears that one-jar cannot be made _completely_ silent :/

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Remove one useless (and not working) instruction in build.gradle

Useless because one-jar does not obey it.

However, this did not make it into the manifest which it should have according
to the docs... Unless I have misread it.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Main program: more random code

Getting there slowly...

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>