BUG: diff on a DataFrame with a datelike broken
```
In [10]: df = DataFrame(dict(time = [Timestamp('20130101 9:01'),Timestamp('20130101 9:02')],value=[1.0,2.0]))

In [11]: df
Out[11]: 
                 time  value
0 2013-01-01 09:01:00      1
1 2013-01-01 09:02:00      2
```

works on series

```
In [12]: df.time.diff()
Out[12]: 
0        NaT
1   00:01:00
Name: time, dtype: timedelta64[ns]
```

Broken with a datelike dtype inclded

```
In [13]: df.diff()
ValueError: cannot include dtype 'm' in a buffer
```
