Allow for downloading Voters & ballots for audit purposes

[sigmavirus24]

As part of running a large public election (e.g., for a public foundation) it'd be nice to be able to download the list of voters and whether or not they cast a ballot. This would help with auditing an election after the fact. Would it be possible to create a CSV and download that?

If I remember correctly, you run on Heroku, so this would require being able to upload to a separate file storage service (e.g., S3).

[joaomorenorf]

I think this issue is a duplicate of #57