download.py: use producer/consumer to reduce memory usage, closes #129.

download.py: use producer/consumer to reduce memory usage, closes #129.
If the DownloadTarget is also an IConsumer, give it control of the brakes
by offering ourselves to target.registerProducer(). When they tell us to
pause, set a flag, which is checked between segment downloads and decodes.
webish.py: make WebDownloadTarget an IConsumer and pass control along to
the http.Request, which already knows how to be an IConsumer.
This reduces the memory footprint of stalled HTTP GETs to a bare minimum,
and thus closes #129.