Version the tutorial

[jkarni]

It would be nice to have multiple versions of the tutorial, corresponding to the major versions of servant. Preliminary work here.

The way I'd propose to do this:

• Make separate branches for each version (e.g. versions/0.4, versions/0.5, etc.).
• Make a separate branch for non-tutorial changes.
• Make the deploy.sh script check what branch it's on.
  • If one of the version branches, then copy over only the tutorial content into an appropriate version subdir (tutorial/0.4/somecontent.html).
  • If the non-tutorial branch, only copy content outside of tutorial folder.
• Redirect tutorial/somecontent.html to tutorial/<latest>/somecontent.html.
• Make sure links still work locally and on travis.