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. :)
Maybe the next iteration? If the bin runs with `--verbose` and returns no error code, then the binary was installed as valid and can read the config. We could add a test for reporting certain Scala syntax errors, but that's a concern of the Java code, not this Ruby install script, eh?