numeric_only inconsistency with pandas Series
```
In [1]: import pandas as pd

In [2]: pd.Series([1,2,3]).sum(numeric_only=False)
Out[2]: 6

In [3]: pd.Series([1,2,3]).sum(numeric_only=True)
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-3-2c46bd289e26> in <module>()
----> 1 pd.Series([1,2,3]).sum(numeric_only=True)

/users/is/whughes/pyenvs/research/lib/python2.7/site-packages/pandas-0.16.2_ahl1-py2.7-linux-x86_64.egg/pandas/core/generic.pyc in stat_func(self, axis, skipna, level, numeric_only, **kwargs)
   4253                                               skipna=skipna)
   4254                 return self._reduce(f, name, axis=axis,
-> 4255                                     skipna=skipna, numeric_only=numeric_only)
   4256             stat_func.__name__ = name
   4257             return stat_func

/users/is/whughes/pyenvs/research/lib/python2.7/site-packages/pandas-0.16.2_ahl1-py2.7-linux-x86_64.egg/pandas/core/series.pyc in _reduce(self, op, name, axis, skipna, numeric_only, filter_type, **kwds)
   2081             if numeric_only:
   2082                 raise NotImplementedError(
-> 2083                     'Series.{0} does not implement numeric_only.'.format(name))
   2084             return op(delegate, skipna=skipna, **kwds)
   2085 

NotImplementedError: Series.sum does not implement numeric_only.
```
