Given a date-indexed Series s, return the mean of the values grouped by calendar month. Index the result by the last calendar day of each month.

Sample data: s:

2015-01-01    0
2015-01-02    1
2015-01-05    2
2015-01-06    3
2015-01-07    4
2015-01-08    5
Freq: B
... (261 rows total)