Missing function Series.day_of_month

[skyqrose]

Here are the current datetime Series functions, added in #624 and #717

• day_of_week/1
• day_of_year/1
• hour/1
• minute/1
• month/1
• second/1
• week_of_year/1
• year/1

It's missing a day_of_month function, which I want to use to turn some datetimes into separate year/month/day columns.

[skyqrose]

Completed by #1134