pandas.rolling_std() first value is nan
The window is 3, but we want a std at `min_periods=1`.  The one-period standard deviation is trivially 0.

```
In [28]: pandas.rolling_std(np.array([1,2,3,4,5], dtype='double'), 3, min_periods=1)
Out[28]: array([        nan,  0.70710678,  1.        ,  1.        ,  1.        ])
```

The pathological case:

```
In [29]: pandas.rolling_std(np.array([1,2,3,4,5], dtype='double'), 1, min_periods=1)
Out[29]: array([ nan,  nan,  nan,  nan,  nan])
```

Maybe it's because pandas is taking the unbiased std for `N-1` where `N = 1`, so it's dividing by zero?
