ENH: .drop_duplicates along the columns axis (axis=1)
#### Code Sample

```python

import pandas as pd
df = pd.DataFrame({'tms': pd.date_range('2017-1-1', '2017-1-31'), 'v': xrange(31)})
df_3 = pd.concat([df, df, df], axis=1)
# What I would like to do: df_3.drop_duplicates(axis=1)
deduplicated_df = df_3.T.drop_duplicates().T
```
#### Problem description

Duplicate columns can be introduced when using the `pd.concat(axis=1)` method.

It would be nice (not necessary) to use the `drop_duplicates` method with an `axis` parameter set to `1`. 

This can be implemented right now using: `df.T.drop_duplicates().T`. Maybe there is an alternative, better way.

Notice that this isn't a  bug but a feature request/feature enhancement of an existing one.

#### Output of ``pd.show_versions()``

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Darwin
OS-release: 16.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: None
Cython: None
numpy: 1.13.0
scipy: 0.19.1
statsmodels: 0.6.1
xarray: None
IPython: 4.0.2
sphinx: None
patsy: 0.4.0
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 2.0.2
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: 0.9.2
apiclient: 1.4.2
sqlalchemy: 1.1.11
pymysql: None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.6
boto: 2.47.0
pandas_datareader: None
```