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.

@josham looks gr8.

after I wrote up the issue I realized that this should prob be named `daysinmonth` (even though I like `days_in_month` better), for compat with the other field naming schema.

we _could_ change these to a proper naming convention, but that's a different issue.
 see #9606 
actually might as well simply add both `days_in_month/daysinmonth` and call it a day. (and maybe just leave the tests alone).
 OK, I'll add daysinmonth
 thanks @josham 

keep em coming!
