BUG: what should we do about illegal operations on mixed frames?
so min across the columns is clearly non-sensical,
what should we do about this when an op crosses dtype boundaries and is wrong ?
(this is harder than it looks, because you can't always just get the numeric data,
a frame with only say timedelta64[ns] is ok) 

maybe have a raise_on_error parameter to min? (default of True)?
(if its mixed (and not all numeric)) types?

```
(Pdb) mixed
                 A                 B    C  D  E
0         00:05:05 -1 days, 00:00:00  foo  1  1
1 1 days, 00:05:05 -1 days, 00:00:00  foo  1  1
2 2 days, 00:05:05 -1 days, 00:00:00  foo  1  1
(Pdb) mixed.min()
A            0:05:05
B    -1 day, 0:00:00
C                foo
D                  1
E                  1
Dtype: object
(Pdb) mixed.min(axis=1)
0   -8.640000e+13
1   -8.640000e+13
2   -8.640000e+13
Dtype: float64
(Pdb) mixed.dtypes
A    timedelta64[ns]
B    timedelta64[ns]
C             object
D              int64
E            float64
Dtype: object
```
