Implement pd.Series.__contains__ instead of pd.Series.isin ?
#### Current Usage

```python
movies[(movies.duration >= 200) & (movies.genre.isin(['Drama', 'Action']))]
```
=>

#### Problem description

v 0.18 introduces the pd.Series.isin method.
Would it be more appropriate to implement this as a `__contains__` method 
so boolean query filter expressions used with data frames can use the 
`in` operator as well?


#### Expected Usage

```python
movies[(movies.duration >= 200) & (movies.genre in ['Drama', 'Action'])]
```

#### Output of ``pd.show_versions()``
<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Darwin
OS-release: 16.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.1
pytest: None
pip: 9.0.1
setuptools: 35.0.2
Cython: None
numpy: 1.12.1
scipy: 0.17.0
xarray: None
IPython: 5.3.0
sphinx: 1.5.5
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
