attendee newsletter CSV export

creates a CSV file that is ready to import into MadMimi (and probably other providers) of attendees who want to sign up for the newsletter

at some future point I will also do this with staff newsletter people as well.

need Eli's eyes on this before merge, appears to work correctly on my machine.
Instead of ``filter(Attendee.can_spam == True)`` you can use the shorter version ``.filter_by(can_spam=True)`` which does exactly the same thing, so it's purely a stylistic choice.  I tend to prefer the shorter version and I'd been using SQLAlchemy for like a year before I realized it existed, so I figured it was worth mentioning.

Side node: does MadMimi really care about zip code?