ENH/BUG: Period/PeriodIndex supports NaT

Closes #7228. Closes #4731.

`Period` and `PeriodIndex` now can contain `NaT` using `iNaT` as its internal value.


Because this is taking elemental-wise logical or. This is for the case like
```
idx1 = pd.PeriodIndex(['2011-01', 'NaT'], freq='M')
idx2 = pd.PeriodIndex(['NaT', '2011-01'], freq='M')
idx1 > idx2
# [False False]
```