Initial sketch of bug #282, aggregate cell data export.

I believe this mostly accomplishes the "hourly exports of changes to Cell table" issue, in very broad brush strokes. I haven't managed to test it against S3 live yet and I expect the fields will change some and possibly need preprocessing before export, but I'm posting this for feedback / review now.
This class and the selfdestruct_tempdir might be candidates for the top level util module (if they are re-used across the backup and export modules).