ENH/BUG: Period/PeriodIndex supports NaT

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

?
```
In [56]: idx1 = pd.PeriodIndex(['2011-01', '2011-02'], freq='M')

In [59]: idx2 = pd.PeriodIndex(['2011-01', '2011-03'], freq='M')

In [60]: idx1>idx2
Out[60]: array([False, False], dtype=bool)

In [61]: idx2>idx1
Out[61]: array([False,  True], dtype=bool)
```