API: way to exclude the grouped column with apply
http://stackoverflow.com/questions/23709811/best-way-to-sum-group-value-counts-in-pandas/23712433?noredirect=1#comment36441762_23712433
any of these look palatable?

`df.groupby('c',as_index='exclude').apply(....)`
`df.groupby('c')['~c'].apply(...)`
`df.groupby('c',filter='c')`
`df.groupby('c',filter=True)`

rather than doing a negated selection (`df.columns-['c']`)

```
df = pd.DataFrame.from_dict({'b1':['aa','ac','ac','ad'],\
                             'b2':['bb','bc','ad','cd'],\
                             'b3':['cc','cd','cc','ae'],\
                             'c' :['a','b','a','b']})

df.groupby('c')[df.columns - ['c']].apply(lambda x: x.unstack().value_counts())
Out[92]: 
c    
a  cc    2
   bb    1
   ad    1
   ac    1
   aa    1
b  cd    2
   ad    1
   ae    1
   ac    1
   bc    1
dtype: int64
```
