ENH: allow datelike to be aggregated in resample
http://stackoverflow.com/questions/21878599/pandas-drops-timestamp-columns-in-resample

```
init_time=pd.date_range(start='2010-01-01 00:00', end='2010-01-05 00:00', freq='12H')
valid_time=pd.date_range(start='2010-01-01 00:00', end='2010-01-05 00:00', freq='12H')
data = np.random.random(len(valid_time))
frame = pd.DataFrame(index=valid_time, data=data)
frame['init_time'] = init_time
```

Works, but a little hacky

```
In [48]: numeric = frame.resample('D',how='mean')

In [49]: datelike = frame.loc[:,frame.dtypes.isin([np.dtype('datetime64[ns]')])]

In [50]: datelike = datelike.resample('D',
         how=lambda x: Timestamp(x.view('i8').mean()))

In [51]: concat([numeric,datelike],axis=1)
Out[51]: 
                   0           init_time
2010-01-01  0.798880 2010-01-01 06:00:00
2010-01-02  0.859781 2010-01-02 06:00:00
2010-01-03  0.515503 2010-01-03 06:00:00
2010-01-04  0.505557 2010-01-04 06:00:00
2010-01-05  0.979835 2010-01-05 00:00:00

[5 rows x 2 columns]
```
