Async fixture downloads

http://manage.dimagi.com/default.asp?145164

![screenshot 2014-11-12 08 06 04](https://cloud.githubusercontent.com/assets/464726/5010675/993cfa1e-6a43-11e4-9695-ce6421853723.png)

![screenshot 2014-11-12 08 07 42](https://cloud.githubusercontent.com/assets/464726/5010681/a1681d22-6a43-11e4-9bfc-665766e5602a.png)

The file download part of the code was split out into this function. This is mostly a direct copy of the end of the old `views.py:download_item_lists`.