ENH: EWM Sum?
I _think_ this would be just: `.ewm(span).mean() * span`

Now:

``` python

In [4]: pd.Series(range(10)).ewm(span=2).sum()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-8e0e72f35eb3> in <module>()
----> 1 pd.Series(range(10)).ewm(span=2).sum()

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/core/window.py in __getattr__(self, attr)
    122 
    123         raise AttributeError("%r object has no attribute %r" %
--> 124                              (type(self).__name__, attr))
    125 
    126     def _dir_additions(self):

AttributeError: 'EWM' object has no attribute 'sum'
```

CC @erbian
