ENH: Add days_in_month property to Timestamp/DatetimeIndex/... (GH9572)
closes #9572 

Added days_in_month property (compatibility alias daysinmonth) to Timestamp, DatetimeIndex, Period, PeriodIndex,
Series.dt. 

Although not mentioned in the GH issue, I added the property to Period and PeriodIndex to be consistent with the others: dayofweek, dayofyear, etc.
