I suggest a radially new approach with `crosstab()` from the additional module tablefunc. You need to install it once per database. Detailed instructions:

 - [PostgreSQL Crosstab Query][1]

</p>

    SELECT k.name, keyword_id, project_id, now()::date AS the_date
         , t AS today, y As yesterday, w AS week, m AS month
    FROM   crosstab(
       $$
       SELECT rn
            , pk.keyword_id
            , pk.project_id
            , d.created_at
            , COALESCE(pr.pos, 0) AS pos
       FROM  (SELECT *, row_number() OVER () AS rn FROM project_keyword) pk
       CROSS  JOIN (
          VALUES
             (now()::date)
           , (now()::date - 1)
           , (now()::date - 7)
           ,((now() - interval '1 month')::date)
         ) d(created_at)
       LEFT JOIN (
          SELECT keyword_id
               , project_id
               , created_at::date AS created_at
               , min(position) AS pos
          FROM   project_report
          GROUP  BY keyword_id, project_id, created_at::date
          ) pr USING (keyword_id, project_id, created_at)
       ORDER  BY pk.rn, d.created_at
       $$
    
      ,$$
       VALUES
             (now()::date)
           , (now()::date - 1)
           , (now()::date - 7)
           , ((now() - interval '1 month')::date)
       $$
       ) ct (rn int, keyword_id int, project_id int
           , t int, y int, w int, m int)
    JOIN   keyword k USING (keyword_id);

Returns:

    name    keyword_id   project_id   the_date     today   yesterday   week   month
    Cheese  1            1            2014-07-11   1       1           1      0
    Monitor 2            1            2014-07-11   2       2           2      0
    Apple   3            1            2014-07-11   9       9           9      0
    Apple1  4            1            2014-07-11   0       0           0      0
    Iphone  5            1            2014-07-11   0       33          34     0

###Explain

 - We need a single unique column per group for `crosstab()`. Since you don't have that per `(keyword_id int, project_id)` I am adding a surrogate key with `row_number()` to `project_keyword`.
 - JOIN that to a set of dates you are interested in (today, yesterday, ...). One row per date.

 - LEFT JOIN to `project_reports` for said days.

 - Cross-tabulate to get one column with the position per date.

 - Finally JOIN to `keyword` just to add the name. Add the current date and remove `rn` from the result while being at it.

Can't provide a fiddle, installing additional modules is not allowed there.

  [1]: http://stackoverflow.com/questions/3002499/postgresql-crosstab-query/11751905#11751905