bumped three.js to r66
new slides for varnish 4
finally got rid of the json files

The pain of json hand-writing is over now that data files are written in
yaml. The bonus is that json files are also minified too. It makes more
requirements for building the presentation, but this really is such a
tiny price :)

The `install-check` target was renamed to `env-check` since the former
felt a bit confusing.
various improvements in the code
added a `beforeunload` listener
the breizhcamp presentation is now available
added a short index page linking to both talks
updated readme and got rid of CC licensing
s/back-end/backend/