basic zip download implementation

- config.ini can be used to activate the download option and set the caption for the download button
- a zip archive will be created in /.zip/myFolder.zip unless it exites (NO updates)
- a download button is displayed at the end of the filelist

- Bugfixes:
  - INSTALL: Forgotten folder to link
  - Thumbnail.class.php: set file extention (.jpg) to the end
  - DirectoryListing.class.php: rawurlencode([path.file]) led to encoded slashes