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)

Changed this to use `StringIO`. The previous implementation created a temp file that was never cleaned up. This seems more efficient: skip write to disk, same memory footprint.