Allow stopping/pausing torrents

When writing my poor and dirty client I encountered a serious problem: your library by default is not able to stop/pause torrents, only remove them with `Drop()` method. I was able to overcome this by exporting [close](https://github.com/anacrolix/torrent/blob/master/torrent.go#L548) method and adding additional [bit of code](https://github.com/ecdsa521/gourmet/blob/master/lib-torrent-patch.diff) and calling it before `DownloadAll()`, and while it seems to work, I am certain this could be done better, and some torrents sometimes do not want to start again - are stuck at 0 peers/seeds.

So, let it be feature request: Please let us stop torrents in a sane, supported way.

Thanks for the great lib btw.