Make executable jar work better as an embeddable

I think it's a good idea to provide an executable jar (good
getting started experience), but it wasn't working very well.
The biggest problem was the missing main() method, but that
was easy to fix. More interestingly I have changed the default
config name to "configserver" so "configserver.yml" is loaded
to set the default config for the standalone jar, and this file
can easily be ignored in a customized server.

Fixes gh-17