Computing time difference with timezone in Series with one row gives wrong result
When there are only one row containing timestamps with timezone in DataFrame, computing time difference between columns gives incorrect result - always '0 days':

```
In [133]: import pandas as pd

In [134]: df = pd.DataFrame(columns=['s', 'f'])

In [135]: df.s = [pd.Timestamp('2016-02-08 13:43:14.605000', tz='America/Sao_Paulo')]

In [136]: df.f = [pd.Timestamp('2016-02-10 13:43:14.605000', tz='America/Sao_Paulo')]

In [137]: df
Out[137]:
                                 s                                f
0 2016-02-08 13:43:14.605000-02:00 2016-02-10 13:43:14.605000-02:00

In [138]: df.f - df.s    # should give '2 days'
Out[138]:
0   0 days
Name: f, dtype: timedelta64[ns]

In [139]: df.s - df.f    # should give '-2 days'
Out[139]:
0   0 days
Name: s, dtype: timedelta64[ns]
```

If there are more than one row, result is correct:

```
In [140]: import pandas as pd

In [141]: df = pd.DataFrame(columns=['s', 'f'])

In [142]: df.s = [pd.Timestamp('2016-02-08 13:43:14.605000', tz='America/Sao_Paulo'), pd.Timestamp('2016-02-08 13:43:14.605000', tz='America/Sao_P aulo')]

In [143]: df.f = [pd.Timestamp('2016-02-10 13:43:14.605000', tz='America/Sao_Paulo'), pd.Timestamp('2016-02-11 15:43:14.605000', tz='America/Sao_P aulo')]

In [144]: df
Out[144]:
                                 s                                f
0 2016-02-08 13:43:14.605000-02:00 2016-02-10 13:43:14.605000-02:00
1 2016-02-08 13:43:14.605000-02:00 2016-02-11 15:43:14.605000-02:00

In [145]: df.f - df.s
Out[145]:
0   2 days 00:00:00
1   3 days 02:00:00
Name: f, dtype: timedelta64[ns]
```

One row containing timestamps in naive datetime also gives correct result:

```
In [147]: import pandas as pd

In [148]: df = pd.DataFrame(columns=['s', 'f'])

In [149]: df.s = [pd.Timestamp('2016-02-08 13:43:14.605000')]

In [150]: df.f = [pd.Timestamp('2016-02-10 13:43:14.605000')]

In [151]: df
Out[151]:
                        s                       f
0 2016-02-08 13:43:14.605 2016-02-10 13:43:14.605

In [152]: df.f - df.s
Out[152]:
0   2 days
dtype: timedelta64[ns]
```

Installed versions:

```
In [162]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-49-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.1
nose: None
pip: 8.0.2
setuptools: 18.2
Cython: None
numpy: 1.10.4
scipy: None
statsmodels: None
IPython: 4.0.3
sphinx: None
patsy: None
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: 0.9.2
apiclient: 1.4.2
sqlalchemy: None
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
Jinja2: None
```
