API: groupby.transform()
```
In [4]: df = pd.DataFrame({'A':[1,1,2,2],'B':[1,2,3,4]})

In [5]: df.groupby('A').transform('max')
Out[5]: 
   B
0  2
1  2
2  4
3  4
```

Might be nice to have a deferred ``.transform()`` object (notice it doesn't take any args).
```
In [7]: df.groupby('A').transform().max()
```

Pretty sure this could be back-compat.