Overhaul of the ftp_download plugin

* Big overhaul of code.
* Bugfixes, among other it started to dl entire server if origin
path/file doesn't exists.
* Fixed TODO ''resume downloads''
* Fixed TODO ''create banlist files'' --> skiplist config option.
* Added support for secure dl --> use-secure config option
* Converted to config_schema-type config definition.
* Changed config setting: ftp_tmp_path -> path (more clear)