ENH: Allow aggregate numeric operations on timedelta64.
closes #5724

Currently, `timedelta64` columns can't be used in aggregate group operations. They are quietly dropped or, if no other columns are present, an exception is raised:

```
In [2]: df
Out[2]: 
     A        E
0  foo 00:00:01
1  bar 00:00:02
2  foo 00:00:03
3  bar 00:00:04


In [3]: df.groupby('A').mean()
Out[3]: 
(...)
DataError: No numeric types to aggregate
```

But any operation that works on numerical data is also well defined on timedeltas. This PR relaxes the restriction on those methods designated "numeric only" to accept numeric dtypes _and_ `timedelta64`. I include a unit test with one simple case each for `DataFrameGroupBy` and `SeriesGroupBy`.

```
In [2]: df.groupby('A').mean()
Out[2]: 

           E
A           
bar 00:00:03
foo 00:00:02
```
