Payday rewrite, part 5

Step 6 of #2508. Follow-on from #2541.
May seem pedantic, but I [*would*](https://github.com/gittip/www.gittip.com/pull/2579#issuecomment-50657409) like to change the temporary table names from `pay_` to `payday_`. Fits our existing nomenclature better.