Timedeltaindex as columns + groupby fails
``` python
import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.normal(size=(10,15)))
df.columns = pd.timedelta_range(start='0s',periods=15,freq='1s')
df.groupby(['a']*5+['b']*5)
```

fails (throwing ValueError), whereas

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

doesn't.

```
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)
```
