Can pd.DataFrame retain column order when passed in OrderedDict?
It would have the nice effect of having

``` python

ohlc = OrderedDict()
ohlc['open'] = 'first'
ohlc['high'] = 'max'
ohlc['low'] = 'min'
ohlc['close'] = 'last'

gdf = grouped.agg(ohlc)
```

come out with the proper column order. 

core/groupby.py

``` python
1593             else:                                                                               
1594                 result = DataFrame(result) 
```

result is ordered correctly before being passed to DataFrame constructor. 
