cumsum(skipna=True) bug if last element is NaN
From the documentation example
http://pandas.pydata.org/pandas-docs/stable/basics.html#descriptive-statistics

For the DataFrame df in the example:

```
In [39]: df

        one     three       two
a -0.701368       NaN -0.087103
b  0.109333 -0.354359  0.637674
c -0.231617 -0.148387 -0.002666
d       NaN -0.167407  0.104044
```

the example for cumsum() seems to expose a potential bug when the last element is NaN (d,one). Notice that the output from cumsum() have nan for (d,one) entry. This should be same as (c,one) value. 

```
In [48]: df.cumsum()

        one     three       two
a -0.701368       NaN -0.087103
b -0.592035 -0.354359  0.550570
c -0.823652 -0.502746  0.547904
d       NaN -0.670153  0.651948
```

Am I missing something? 
Thanks,
R
