BUG: ufunc with PeriodIndex may raise IncompatibleFrequency
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`PeriodIndex` supportds addition / subtraction with integer.

```
# OK
pi = pd.PeriodIndex(['2011-01'], freq='M')
pi + 1
# PeriodIndex(['2011-02'], dtype='int64', freq='M')
pi - 1
# PeriodIndex(['2010-12'], dtype='int64', freq='M')
```

but raises if op is performed via ufunc.

```
# NG
np.add(pi, 1)
# pandas._period.IncompatibleFrequency: Input has different freq from PeriodIndex(freq=M)

np.subtract(pi, 1)
# pandas._period.IncompatibleFrequency: Input has different freq from PeriodIndex(freq=M)
```
