New release with backwards-incompatible changes

[zorun]

I would to do a number of changes that would break compatibility with existing installations. Nothing bad, but configuration would need to be adapted. So we would need to do a "release" and communicate the changes.

For now I'm thinking about:

• simplify configuration syntax: lg: Change configuration of proxied hosts (not backward compatible) #75
• add support for python3 and remove support for python2 (which is no longer supported)
• move config files to lg.cfg.example and lgproxy.cfg.example so that actual configuration files can be managed independently of the git repo

If you have other ideas for changes that would break compatibility, it's a good time so that we "release" everything at once.