Clean up torrent creation methods on Client

Per https://github.com/anacrolix/torrent/issues/243#issue-310328823:

> remove torrent creation methods from the client. there are just want too many of them. poorly documented, and its unclear which one should be used and when.
> ```
> func (cl *Client) AddMagnet(uri string) (T *Torrent, err error) {}
> func (cl *Client) AddTorrentFromFile(filename string) (T *Torrent, err error) {}
> func (cl *Client) AddTorrent(mi *metainfo.MetaInfo) (T *Torrent, err error) {}
> func (cl *Client) AddTorrentInfoHash(infoHash metainfo.Hash) (t *Torrent, new bool) {}
> func (cl *Client) AddTorrentInfoHashWithStorage(infoHash metainfo.Hash, specStorage storage.ClientImpl) (t *Torrent, new bool) {}
> func (cl *Client) AddTorrentSpec(spec *TorrentSpec) (t *Torrent, new bool, err error) {}
> ```