Update arte-downloader.user.js

- remove dead code.
- fix #2 by adding support for non Greasmonkey user scripts implementation
- add support for Presto based implementation which yield the fastest arte+7 page loads in all browsers
- removed `triggerOnClick()` which is a dead function.
- fix a couple of warnings jshint (a javascript static analyzer) spotted.
- add updateURL and downloadURL tags in the header, so the script can update itself if/when new versions come.