Fix #82: Only include Scala.js-emitted .js files in packageJS.

Other .js files on the classpath are not bundled by default by
packageJS (nor optimizeJS). Of course, .js files added explicitly
to (sources in (Compile, packageJS)) are still packaged, e.g.,
the typical startup.js file.
Merge pull request #299 from sjrd/packagejs-only-scalajs-files

Fix #82: Only include Scala.js-emitted .js files in packageJS.