ENH: implement 'reverse' for grouping
when there are lots of columns of index levels it is more convenient to specify the column or index level that should be merged rather than all of the other columns or index levels

for example a multiindex dataframe

```
df.mean(level=['year', 'month', 'day', 'hour', 'min', 'trial'])
```

can be replaced with

```
df.mean(level='rep', reverse_level=True)
```

or for a flat index dataframe (where the data is in a column named 'result')

```
df.groupby(['year', 'month', 'day', 'hour', 'min', 'trial']).mean()
```

can be replaced with

```
df.groupby(['rep', 'result'], reverse=True)
```

http://stackoverflow.com/questions/16808682/is-there-in-pandas-operation-complementary-opposite-to-groupby
