Fix #1169: Make IR printer print cases of match tree nicely

It now looks like this:

    match (x1) {
      case 1 | 2 | 3:
        mod:s_Predef$.println__O__V("low");
      case 4:
        mod:s_Predef$.println__O__V("four");
      default:
        mod:s_Predef$.println__O__V("other");
    }
Merge pull request #1189 from gzm0/fix-ir-printer-match

Fix #1169: Make IR printer print cases of match tree nicely
Avoid crashing for classpath elements that do not exists on the file system.

Added DummyVersion constant value, exists() to detect file that does not exists and getDummyVersion to provide a string version for a dummy file.
Implemented DummyVersion and isDummy in physical and node file systems.
Added simple ClasspathElementsTraverser tests using the PhysicalFileSystem.
Merge pull request #1167 from oleastre/ignore-file-if-not-exist

Ignore classpath elements when it's neither JS, JAR or directory (probably file does not exists).