Use 'net-ftp-list' gem to parse ftp directory listing.

It also parses non-unix ftp servers, which have a different list format.
Filenames with white spaces break that line:
https://github.com/PetroFeed/huginn/commit/a2243590f659d1a43acc6be3fb8df833497823f5#diff-33e864c7a3d3a3bbfc8687b220883e98R40