Add support for scala-native and scalajs 1.0

[marcesquerra]

In a project I'm working on, I would like to use scala-newtype from both scala-native and scalajs 1.0.

If you find it useful, I've spent a bit of time trying to upgrade scalajs and add support for scala-native into your project.

In summary, this PR does the following:

• Upgrade the version of the related sbt plugins, plus adding the scala-native ones.
• Add the scala-native platform to the core project (not to the cats testing one)
• Add a small bit of configuration specific for the scala-native project
• Upgrades the version of scala test (and adds an extra submodule that in the new version of scalatest lives outside of the main library)
• Extract one test so that it only runs for JVM and JS (doesn't work on native: ConcurrentHashMap has not been implemented there)

[marcesquerra]

I realise now there already is a PR to upgrade scalajs, if you merge that at some point, I'll fix this afterwards :)

[marcesquerra]

Now this show be ready to merge!