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'
```
