Publish documentation on GitHub pages

[guilgaly]

Pull Request Checklist

• Have you read through the contributor guide?
• Have you added tests for any changed functionality?

Purpose

Publish both generated API docs (scaladoc) and a documentation website to Github pages.

Background Context

• The generated Scaladoc for core and for the modules are unified with sbt-unidoc for simplicity, but we could also put them in separate folders.
• The doc site is generated with sbt-site using the paradox static site generator. Paradox is used for sbt-site's own doc site, so we have a nice example there.
• Everything is then pushed to Github Pages using sbt-ghpages.

References

See issue #18

TODO

• Write more documentation...
• Write doc for the documentation writing/publishing process too
• Update README
• Embed/link to the license and the contributing guide
• Automate doc publishing with Travis (see this)