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)

Unless I read wrong, `export_raw` with `Excel2007ExportWriter` does not actually write to the file until `_close` is called. In this case the write to file happened immediately before `file.read()` (now `file.getvalue()`), so at least in this case I don't think it was "streaming to a file" and `file.read()` would immediately bring it all back into memory too.