Why don't you simply do something like this ?
WITH RECURSIVE dates(d)
AS (VALUES('2014-05-01')
UNION ALL
SELECT date(d, '+1 day')
FROM dates
WHERE d < '2014-05-5')
SELECT
dates.d AS date
,table.column1
,table.column2
FROM dates
left join table
ON strftime("%Y-%m-%d", table.date_column) = dates.date
Perhaps you will need to convert your date...