Groupby built by columns : cannot use .head() or .apply()
``` python
import numpy as np
import pandas as pd

df = pd.DataFrame({i:pd.Series(np.random.normal(size=10),
                                index=range(10)) for i in range(11)})

df_g = df.groupby(['a']*6+['b']*5, axis=1)
```

This, if I well understood, should build a groupby object grouping columns, and so give the possibility to later aggregate them. And indeed :

``` python
df_g.sum()
```

works well. But

``` python
df_g.head()
```

Throws an error:

```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jonas/Code/pandas/pandas/core/groupby.py", line 986, in head
    in_head = self._cumcount_array() < n
  File "/home/jonas/Code/pandas/pandas/core/groupby.py", line 1044, in _cumcount_array
    cumcounts[indices] = values
IndexError: index 10 is out of bounds for axis 1 with size 10
```

and

``` python
df_g.apply(lambda x : x.sum())
```

from which I expected the same result as the first example, gives this table :

```
           a         b
0  -0.381070       NaN
1  -1.214075       NaN
2  -1.496252       NaN
3   3.392565       NaN
4  -0.782376       NaN
5   1.306043       NaN
6        NaN -1.772334
7        NaN  4.125280
8        NaN  1.992329
9        NaN  4.283854
10       NaN -4.791092
```

I didn't really get what's happening, I don't exclude a misunderstanding or an error from myself.

```
pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.8.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-46-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8

pandas: 0.16.0-28-gcb8c130
nose: 1.3.4
Cython: 0.20.2
numpy: 1.9.2
scipy: 0.14.0
statsmodels: None
IPython: 3.0.0-dev
sphinx: 1.2.2
patsy: None
dateutil: 2.4.1
pytz: 2015.2
bottleneck: None
tables: 3.1.1
numexpr: 2.4
matplotlib: 1.4.3
openpyxl: 1.7.0
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 0.9.7
pymysql: None
psycopg2: 2.5.3 (dt dec mx pq3 ext)
```
