Invalid comparison of object Series objects containing None
In pandas 0.15.2:

```
from pandas import Series
a = Series(['A', None])
b = Series(['A', None])
a == b
```

gives

```
0     True
1    False
dtype: bool
```

while `a[1] == b[1]` gives `True`. This is a) inconsistent and b) the behaviour of `a == b` is wrong (None is equal to None, so `a==b` should return `[True, True]`.
