groupby.mean, etc, doesn't recognize timedelta64
See http://stackoverflow.com/questions/20625982/split-apply-combine-on-pandas-timedelta-column
related as well: http://stackoverflow.com/questions/20789976/python-pandas-dataframe-1st-line-issue-with-datetime-timedelta/20802902#20802902

I have a DataFrame with a column of timedeltas (actually upon inspection the dtype is `timedelta64[ns]` or `'<m8[ns]'`), and I'd like to do a split-combine-apply, but the timedelta column is being dropped:

``` python
import pandas as pd

import numpy as np

pd.__version__
Out[3]: '0.13.0rc1'

np.__version__
Out[4]: '1.8.0'

data = pd.DataFrame(np.random.rand(10, 3), columns=['f1', 'f2', 'td'])

data['td'] *= 10000000

data['td'] = pd.Series(data['td'], dtype='<m8[ns]')

data
Out[8]: 
         f1        f2              td
0  0.990140  0.948313 00:00:00.003066
1  0.277125  0.993549 00:00:00.001443
2  0.016427  0.581129 00:00:00.009257
3  0.048662  0.512215 00:00:00.000702
4  0.846301  0.179160 00:00:00.000396
5  0.568323  0.419887 00:00:00.000266
6  0.328182  0.919897 00:00:00.006138
7  0.292882  0.213219 00:00:00.008876
8  0.623332  0.003409 00:00:00.000322
9  0.650436  0.844180 00:00:00.006873

[10 rows x 3 columns]

data.groupby(data.index < 5).mean()
Out[9]: 
             f1        f2
False  0.492631  0.480118
True   0.435731  0.642873

[2 rows x 2 columns]
```

Or, forcing pandas to try the operation on the 'td' column:

``` python
data.groupby(data.index < 5)['td'].mean()
---------------------------------------------------------------------------
DataError                                 Traceback (most recent call last)
<ipython-input-12-88cc94e534b7> in <module>()
----> 1 data.groupby(data.index < 5)['td'].mean()

/path/to/lib/python3.3/site-packages/pandas-0.13.0rc1-py3.3-linux-x86_64.egg/pandas/core/groupby.py in mean(self)
    417         """
    418         try:
--> 419             return self._cython_agg_general('mean')
    420         except GroupByError:
    421             raise

/path/to/lib/python3.3/site-packages/pandas-0.13.0rc1-py3.3-linux-x86_64.egg/pandas/core/groupby.py in _cython_agg_general(self, how, numeric_only)
    669 
    670         if len(output) == 0:
--> 671             raise DataError('No numeric types to aggregate')
    672 
    673         return self._wrap_aggregated_output(output, names)

DataError: No numeric types to aggregate
```

However, taking the mean of the column works fine, so numeric operations should be possible:

``` python
data['td'].mean()
Out[11]: 
0   00:00:00.003734
dtype: timedelta64[ns]
```
