Period.end_time is incorrect when period is W or longer
For example:

> > > p = pandas.Period('2012', freq='M')
> > > p.end_time
> > > <Timestamp: 2012-01-31 00:00:00>

This is the _start_ of the last day of the month.  So if one were using this in code, you could erroneously find that any datetime on January 31st is after the end of the month of January, for example:

> > > datetime.datetime(2012, 1, 31, 12) > p.end_time
> > > True
