API: correctly provide __name__ for cum functions
closes #12021

```
(py3.5)[Tue Apr 05 08:16:18 ~/miniconda/envs/py3.5/pandas]$ ipython
Python 3.5.1 |Continuum Analytics, Inc.| (default, Dec  7 2015, 11:24:55) 

In [1]: pd.Series.sum.__qualname__
Out[1]: 'Series.sum'

In [2]: pd.Series.cumsum.__qualname__
Out[2]: 'Series.cumsum'

In [3]: pd.Series.cumsum.__module__
Out[3]: 'pandas.core.series'

In [4]: pd.Series.cumsum.__name__
Out[4]: 'cumsum'
```

cc @mrocklin
 0.18.0

```
In [1]: Series.sum.__qualname__
Out[1]: '_make_stat_function.<locals>.stat_func'
```

this PR

```
In [1]: Series.sum
Out[1]: <function pandas.core.series.Series.sum>

In [2]: Series.sum.__qualname__
Out[2]: 'Series.sum'

In [3]: Series.cumsum.__qualname__
Out[3]: 'Series.cumsum'
```

cc @mrocklin
this doesn't solve the `cloudpickle` issue, but I think makes progress
 cc @mrocklin did you have a chance to try this? 
 Ah, sorry for letting this slip
