Floor division by zero result differs if index unaligned
Depending on if the indices of two series/dataframes are aligned, floor division by zero can result in either `NaN` or `inf`:

``` python
In [68]: import pandas as pd

In [69]: l = pd.Series([1, 2, 3], index=[1, 2, 3])

In [70]: r = pd.Series([1, 1, 0], index=[0, 2, 3])

In [71]: l // r    # Unaligned indices
Out[71]:
0    NaN
1    NaN
2    2.0
3    NaN
dtype: float64

In [72]: l.iloc[1:] // r.iloc[1:]    # Aligned indices
Out[72]:
2    2.000000
3         inf
dtype: float64

In [73]: pd.__version__
Out[73]: u'0.18.0'
```

I would expect the result for index 3 to be the same in either case. In pandas 0.17.1, both result in `inf`.
