scalastyle 0.5.0 (new formula)

Scalastyle is a style checker for Scala. They have a CLI interface and there
have been requests to make it available as a Homebrew formula:
  https://github.com/scalastyle/scalastyle-batch/issues/1

Here is such a formula. :)
Needs an explicit `version` declaration; Homebrew can't parse what the version string is here.