BUG: iat/at broken for PeriodIndex
```
In [9]: s = Series(randn(10), index=period_range('1/1/2001', periods=10))

In [10]: s
Out[10]:
2001-01-01   -0.677
2001-01-02    0.058
2001-01-03   -1.200
2001-01-04   -1.152
2001-01-05   -2.646
2001-01-06   -1.068
2001-01-07   -1.232
2001-01-08    0.452
2001-01-09    0.476
2001-01-10   -1.387
Freq: D, dtype: float64

In [11]: s.iat[0] # throws a KeyError
```
