Fix 2 problems with Filename completion:
Absolute path completion broken on Windows
path completion adds blank at the end of filenames, which is annoying in groovysh

This copies FileNameCompleter from JLine2.10, possibly one day in the future,
JLine might provided fixes on its own. However having a Groovy-maintained
FileNameCompleter does not hurt.
Merge pull request #203 from tkruse/fix_filename_completer

Fix 2 problems with Filename completion: