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).