mdbook setup and draft of tutorial

[killercup]

First step towards #46

Still todo:

• [x] render this on CI
• [x] add more content

[killercup]

Added CI (with publish to gh-pages when run on master, let's see if it works) and split tutorial into chapters. Also added some demo structure based on https://github.com/rust-lang-nursery/cli-wg/issues/46#issuecomment-408386068.

[killercup]

I see/hear no people screaming at me that this is bad so I'm gonna merge it :)