ENH: add groupby(..).sort_values()
might be useful to expose this as a top-level groupby method for intra-group sorting. Anyone have a good usecase for this? We could make this *much* faster that the current impl (not that this is an issue, though potentially it could be).
```
In [10]: df = DataFrame({'A': [1, 1, 2, 2, 2], 'B': [2, 1, 3, 3, 1], 'C':[1, 1, 2, 1, 1]})

In [11]: df
Out[11]: 
   A  B  C
0  1  2  1
1  1  1  1
2  2  3  2
3  2  3  1
4  2  1  1

In [12]: df.groupby('A').apply(lambda x: x.sort_values(['B', 'C']))
Out[12]: 
     A  B  C
A           
1 1  1  1  1
  0  1  2  1
2 4  2  1  1
  3  2  3  1
  2  2  3  2

```