updated checkouts scraper to return iso date

- I wanted to compare the due date to today for some magical
  data binding voodoo, but the mm/dd/yyyy date format was making
  it difficult and non-lightweight on the client end, so I added
  the 'american_date' gem and added an iso_due_date field to the
  scrape_checkouts def, using Date.parse().to_s.

Signed-off-by: William Rockwood <wrockwood@tadl.org>