Inconsistent Timestamp arithmetic
Subtracting a scalar datetime from a series returns timedeltas, but doesn't do the same for a dataframe.

```
import pandas as pd 
series = pd.Series(pd.date_range('2000-1-1', periods=100), index=range(0, 100))
df = pd.DataFrame(series)
date = pd.Timestamp('1995-1-1')

series - date
#0    1826 days                                                                                 
#1    1827 days                                                                                 
#2    1828 days
# ....   

df - date
#             0
#0  1975-01-01                                                                                  
#1  1975-01-02                                                                                  
#2  1975-01-03
# ....
```

It seems like both should return timedeltas.
