Fix backslash handling in File.expand_path for Windows
fix backslash handling in File.dirname on Windows, and handling of trailing (back-)slashes
Merge remote-tracking branch 'origin/master' into tim/File-Windows-backslash-handling
only accept backslashes as path separators on Windows
make sure we don't go out of bounds
Merge remote-tracking branch 'origin/tim/File-Windows-backslash-handling' into tim/File-Windows-backslash-handling
Merge pull request #573 from topazproject/tim/File-Windows-backslash-handling

Handle backslashes in File.dirname and File.expand_path on Windows