Adding 'download'-Option to config.ini
Baisc download implementation & some bugfixes

- use config.ini to enable zip download and set a caption for the download button
- fix: Thumbnail.class.php: .jpg file extention at the end of the file
- fix: DirectoryListing.class.php: rawurlencode encoded slashes and broke the path
Merge pull request #1 from a-v-l/zipdownload

New feature (download directory contents as zip file) implemented by a-v-l.