keyboard interrupt does not kill the download loop cleanly

[Stuart Mumford]

In a jupyter notebook, because the loop is running in a threadpool, this means that the download isn't cancelled at all. In a regular session it causes all the coroutines to fail with a KeyboardInterrupt exception.

Thoughts on implementing this:

• We can react to the signal with event_loop.add_signal_handler.
• The event handler should cleanly terminate all pending and all running task, by calling Task.cancel (addressing #61 should mean we always have a Task object to cancel).
• This should involve removing any part-finished files from disk so that there aren't corrupt files left lying around.
• When a Task is cancelled it throws a CancelledError. This would need to be caught in _get_http and _get_ftp to cleanly terminate the task.