v1.3: Add an optional image keyword for each codelabs

Each codelabs can now define an optional image keyword to get it staged with
this image reference.
Also replace deployment path due to github being case sensitive now.

Use a new published keyword instead of updated

We moved in the claat tool from "updated" to a manually set "published" keyword. Setting it explicitely helps in particular on the markdown files, as creations and modifications time aren't kept when git cloning them.

That way, we control by updating the published key any creation and major updates. This is valid for both gdoc and markdown files.

Adapt and rebuild against new markdown parser

We changed the markdown parser logic (in the claat package import). Rebuild the binaries to pick those changes.
Adapt tests and generated assets for it.

Also, some minor improvments to avoid disk race between changing assets and sending websocket signal.

v1.0: Adapt site generation to use new Images export path

Adapt tests, generated events and such to follow our new exported path.

v0.9: Hook up websocket to signal page refresh needed

Get a websocket connection ready for any html page opened using the local
server.
For each codelab refreshed, trigger a message sent including the URL so that
client side knows if they need to refresh their page or not. They will only
refresh if the modified tutorial(s) is(are) the current viewed one.

v0.8: Create releases build assets on tag

Autocreate travis-ci build assets everytime a tag is issued.
Upload both generate and serve scripts for autodownload and autorefresh
on the tutorial branch.