DataFrameGroupBy.cumcount() returing Series instead of DataFrame
@hayd 

`DataFrameGroupBy.cumcount()` returns a Series while most others (the ones I tested, like count, cumsum, first, sum ) return a DataFrame. They are maybe not all comparable, but at least with `cumsum` it could be I think:

``` python
In [1]: data = pd.DataFrame({'name' : ['a', 'a', 'b', 'd'], 'counts' : [3,4,3,2]})
In [2]: data
Out[2]:
   counts name
0       3    a
1       4    a
2       3    b
3       2    d

In [3]: g = data.groupby('name')
In [4]: g.cumcount()
Out[4]:
0    0
1    1
2    0
3    0
dtype: int64

In [5]: g.count()
Out[5]:
      counts  name
name
a          2     2
b          1     1
d          1     1

In [6]: g.first()
Out[6]:
      counts
name
a          3
b          3
d          2

In [7]: g.sum()
Out[7]:
      counts
name
a          7
b          3
d          2

In [8]: g.cumsum()
Out[8]:
  counts name
0      3    a
1      7   aa
2      3    b
3      2    d
```
