The sbt plugin must to be published separately

The sbt plugin is not aggregated by the root project anymore, because
the root project is cross-compilable but the sbt plugin is not. Thus to
publish a local version of scala.js you need to do both:

	> publishLocal

and

	> scalajs-sbt-plugin/publishLocal
Merge pull request #329 from jonas/sbt-publish

The sbt plugin must to be published separately