Only include opt-ins in payday script

When a user tips an unclaimed account we create an entry in the
participants table for the tippee, with claimed_time set to NULL.
Tippers by definition must have claimed their account, so we can ignore
unclaimed accounts in the outer payday loop. This will help the numbers
line up better between the output of the payday script and the stats
page. (#37)

gittip/billing.py

@@ -382,6 +382,7 @@ def payday():
  PARTICIPANTS = """\
  SELECT id, balance, stripe_customer_id
  FROM participants
+ WHERE claimed_time IS NOT NULL
  """
  participants = db.fetchall(PARTICIPANTS)
  log("Fetched participants.")