BUG: expanding/rolling_skew/kurt() inconsistent with Series.skew/kurt()
Note that for a constant series, `Series.skew()` returns `0`, while `rolling/expanding_skew()` return `NaN`.

```
In [532]: s = Series([1]*4)

In [533]: s.skew()
Out[533]: 0

In [534]: expanding_skew(s)
Out[534]:
0   NaN
1   NaN
2   NaN
3   NaN
dtype: float64

In [535]: rolling_skew(s, 4)
Out[535]:
0   NaN
1   NaN
2   NaN
3   NaN
dtype: float64
```

While `rolling/expanding_kurt()` similarly return `NaN` for a constant series, `Series.kurt()` produces an error.

```
In [541]: expanding_kurt(s)
Out[541]:
0   NaN
1   NaN
2   NaN
3   NaN
dtype: float64

In [542]: rolling_kurt(s, 4)
Out[542]:
0   NaN
1   NaN
2   NaN
3   NaN
dtype: float64

In [543]: s.kurt()
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-543-e41f2ec435bd> in <module>()
----> 1 s.kurt()

C:\Python34\lib\site-packages\pandas\core\generic.py in stat_func(self, axis, skipna, level, numeric_only, **kwargs)
   3783                                               skipna=skipna)
   3784                 return self._reduce(f, axis=axis,
-> 3785                                     skipna=skipna, numeric_only=numeric_only)
   3786             stat_func.__name__ = name
   3787             return stat_func

C:\Python34\lib\site-packages\pandas\core\series.py in _reduce(self, op, axis, skipna, numeric_only, filter_type, **kwds)
   2007                 filter_type=None, **kwds):
   2008         """ perform a reduction operation """
-> 2009         return op(_values_from_object(self), skipna=skipna, **kwds)
   2010
   2011     def _reindex_indexer(self, new_index, indexer, copy):

C:\Python34\lib\site-packages\pandas\core\nanops.py in _f(*args, **kwargs)
     46                                 'this dtype'.format(f.__name__.replace('nan',
     47                                                                        '')))
---> 48             return f(*args, **kwargs)
     49         return _f
     50

C:\Python34\lib\site-packages\pandas\core\nanops.py in nankurt(values, axis, skipna)
    504     D = _zero_out_fperr(D)
    505
--> 506     result = (((count * count - 1.) * D / (B * B) - 3 * ((count - 1.) ** 2)) /
    507               ((count - 2.) * (count - 3.)))
    508     if isinstance(result, np.ndarray):

ZeroDivisionError: float division by zero
```
