USERID environment variable was no longer passed to docker image in
development mode.
We take the opportunity to improve the documentation regarding the dev
environment.
Close #177.
Replace all make commands by a single "tutor" binary. Environment and
data are all moved to ~/.tutor/local/share/tutor. We take the
opportunity to add a web UI and revamp the documentation.
This is a complete rewrite.
Close #121.
Close #147.
