API: specification of functions in .agg
- [ ] `pd.Summary`
- [x] dict-of-dict #9052
- [ ] named functions #10100 

From [SO](http://stackoverflow.com/questions/26456125/python-pandas-is-order-preserved-when-using-groupby-and-agg/26465555?noredirect=1#comment41593401_26465555)

```
In [100]: df = pd.DataFrame({'A': ['group1', 'group1', 'group2', 'group2', 'group3', 'group3'],
                   'B': [10, 12, 10, 25, 10, 12],
                   'C': [100, 102, 100, 250, 100, 102]})

In [101]: df
Out[101]: 
        A   B    C
0  group1  10  100
1  group1  12  102
2  group2  10  100
3  group2  25  250
4  group3  10  100
5  group3  12  102

In [102]: df.groupby('A').agg(['mean',lambda x: x.iloc[1]])
Out[102]: 
           B               C          
        mean  <lambda>  mean  <lambda>
A                                     
group1  11.0        12   101       102
group2  17.5        25   175       250
group3  11.0        12   101       102
```

Would be nice to be able to use `lambda x: x.nth(1)` or maybe `'nth(1)'`
`In [103]: df.groupby('A').agg(['mean','nth(1')])`
