idxmax() not available on groupby anymore
Just saw an example in this [SO question](http://stackoverflow.com/questions/18879782/python-pandas-groupby-forloop-idxmax), the use of `idxmax()` on a groupby object:

```
df.groupby(...).idxmax()
```

This worked in 0.12, but not anymore in 0.13 as it is not in the whitelist.
- Should this be added to the whitelist? (idxmax/idxmin for SeriesGroupby) I think this is a usefull method on a groupby object.
- I got an AttributeError and not a DeprecationWarning. There was spoken of a deprecation first (#5480), but was it decided not to do it? 
