BUG: Fix .iat indexing with a PeriodIndex GH4390
closes #4390

Interesting fixed this as well (only affects `get`)

```
In [1]: s = Series(randn(10),date_range('20130101',periods=10))

In [2]: s[0]
Out[2]: 1.1755210838393764

In [3]: s
Out[3]: 
2013-01-01    1.175521
2013-01-02    0.007910
2013-01-03   -1.235595
2013-01-04    0.478245
2013-01-05   -0.140616
2013-01-06   -2.317585
2013-01-07    0.609555
2013-01-08    1.204914
2013-01-09   -0.589893
2013-01-10   -2.017237
Freq: D, dtype: float64

In [4]: s[-1]
Out[4]: -2.0172365222181257

In [5]: s[-5]
Out[5]: -2.3175847263447964
```

This was before this PR returning `None` (while `s[-5]` worked)

```
In [6]: s.get(-5)
```

@jreback waiting for @jtratner b4 merging this?
 yep
