Add `scala-compiler` optionally along with `scala-library`

Scala-compiler and scala-library are intimately bound, they must have the same version.
We add the compiler jar optionally to avoid a situation where a dependency needs the
scala-compiler (and therefore the scala-library) for a version lower than `version`,
resulting in a version mismatch.

Note that this change has no tests attached to it. Maybe a scripted test would be nice?
See https://groups.google.com/forum/#!topic/sbt-dev/vUrqem3TfnI