Dump survey data into spreasheet

Adds a feature to dump survey data into a spreadsheet.

Someone who understands databases better than I should probably look over this to see whether and how much I'm being stupid about query performance.  It might not matter, though, since this is the sort of feature that will probably get used a few times per program.

It currently writes in Excel 2003 XML format, because it's human readable and therefore easier to debug.  At some point I might see if I can make it export to a newer format, since LibreOffice chokes on this file for unknown reasons.  (Opening in Excel and resaving as the same format doesn't work, but resaving as a different format does, so I think it's a bug in LO's handling of this format.)

Requires Django 1.4 in at least a couple of places, although if that ends up getting taken out of the stable release for some reason I can probably change that.