groupby TimeStamp bug
#### Code Sample, a copy-pastable example if possible

```
x = pd.DataFrame(index = [1,1,2,2,2], data = pd.date_range("7:00", "9:00", freq="30min", tz = 'US/Eastern'))

x:

1 2016-09-08 07:00:00-04:00
1 2016-09-08 07:30:00-04:00
2 2016-09-08 08:00:00-04:00
2 2016-09-08 08:30:00-04:00
2 2016-09-08 09:00:00-04:00

In [59]: x.groupby(level=0).first()
Out[59]: 
                          0
1 2016-09-08 11:00:00-04:00
2 2016-09-08 12:00:00-04:00
```
#### Expected Output

```
1 2016-09-08 07:00:00-04:00
2 2016-09-08 08:00:00-04:00
```

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-229.14.1.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.1
nose: 1.3.7
pip: 8.1.2
setuptools: 23.0.0
Cython: 0.23.4
numpy: 1.10.2
scipy: 0.16.1
statsmodels: 0.6.1
IPython: 4.0.1
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.5.3
pytz: 2015.7
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.4.6
matplotlib: 1.5.1
openpyxl: 2.2.6
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.7.7
lxml: 3.4.4
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext)
Jinja2: None
```
