Collaboration on Torrent support

Most of the work was done by @mythjuha but it's based on the Completed Download Handling that has _finally_ been merged to develop.

Torrent support integrates nicely with Completed Download Handling since seeding torrents are copied, instead of moved.

Please note that Transmission doesn't support labels, and as such Drone can only import downloads it grabbed explicitly. Unlike uTorrent & Deluge, in which you can add a download to the label and have drone pick it up once finished.

Additionally the GetStatus method on Transmission isn't implemented yet.
We should probably add some prefix ourselves (eg 'utorrent_', to prevent potential conflicts with other download clients. But of course we would have to remove that prefix if used in RemoveDownload.