ENH: support periodindex min/max
related #5202

You can do it but its akward

```
In [142]: pd.period_range('2011-07','2012-05',freq='M').values.min()
Out[142]: 498

In [143]: pd.period_range('2011-07','2012-05',freq='M').values.max()
Out[143]: 508

In [146]: pd.period_range(pd.Period(ordinal=498,freq='M'),pd.Period(ordinal=508,freq='M'),freq='M')
Out[146]: 
<class 'pandas.tseries.period.PeriodIndex'>
freq: M
[2011-07, ..., 2012-05]
length: 11
```

This fails

```
In [150]: pd.period_range('2011-07','2012-05',freq='M').min()
AttributeError: 'PeriodIndex' object has no attribute 'freq'
```
