Make closing of `Downloader`s more concise.

NOTE: This is only a WIP branch in so far as I haven't tested it, as it was done on the train before my holiday. The code is final though, so if people are happy it works, please merge.

The base Downloader class now wraps the InputStream returned by any child classes, in order to notify the child class when that stream is closed. This prevents each child class having to figure out a way to be notified of this.

This helps keep the API concise, because close handling is dealt with without the need to add any public methods to the Downloader class hierarchy.

Also removed some dead code which was unused.