DataFrame.mean() ignores datetime series
#### Code Sample, a copy-pastable example if possible
```python
In [1]: import pandas as pd
In [2]: from datetime import datetime

In [3]: s = pd.Series([datetime(2014, 7, 9), 
   ...:            datetime(2014, 7, 10), 
   ...:            datetime(2014, 7, 11)])

In [4]: df = pd.DataFrame({'numeric':[1,2,3],
   ...:               'datetime':s})

In [5]: df.mean()
Out[5]: 
numeric    2.0
dtype: float64

In [6]: s.mean()
Out[6]: Timestamp('2014-07-10 00:00:00')
```
#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution.]

As of pandas 0.25 it is possible to apply mean() to a datetime series.  However, DataFrame.mean() ignores datetimes series columns rather than returning the mean of the datetime series as one might expect.

#### Expected Output
When axis=0, output could be a dataframe with dtype as the first row, and the value as the second row.

