Torrent data/piece-completion clean-up

The storage implementations(i.e. file) should support clean-up/remove ops as well. Currently even if you `Drop` the torrent you still have to deal with the torrent data removal. So a method such ``RemoveTorrent(infoHash metainfo.Hash)error`` could fix this issue.
This feature is required if you want to develop applications friendly with the peers and at the same time keep the data stored/seeded under your system limits. It would allow us to create various policies (e.g. store and keep active only the latest 3 torrents or torrents of totalling X GB size )