CSV Exporter

Maybe not complete yet, but we could merge it for the sake of not having long running branches and work on the remaining features later. For the record the outstanding things that need to be done:

* Better HTML interface for browsing attendees, including links to orders.
* Better layout of the form for generating a CSV.
* Generation of formats besides CSV.
* Stronger filtering options.

These are not all strictly related to the CSV Exporter (except bullet 2) so maybe it's sufficient to merge now?
Field verbose name should probably default to the model field's verbose name (if their is one.) We could copy Django's api for this part - admin uses method.short_description IIRC for non-field things.