Add an alias 'stable' for 'mergesort' in sort_values
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame({
    'x': [4, 3, 2, 1],
    'y': ['a', 'a', 'a','a']
})

df.sort_values(by='x', kind='mergesort')   # this is the only way to perform a stable-sort
df.sort_values(by='x', kind='stable')      # proposed alias for 'mergesort'
```
#### Problem description

Currently, pandas supports only one way of performing a stable sort i.e. mergesort. Therefore, it makes sense to add an alias for mergesort which will help in making the code more readable.

When I see 'mergesort' written in the code, it is not explicit that this was done to make the sort stable. Therefore having a 'stable' option will fit well with the idea "explicit is better than implicit".


I would like to give this a try if this issue makes sense and the devs decide to accept this proposal.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-37-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_IN
LOCALE: None.None

pandas: 0.20.3
pytest: None
pip: 9.0.2
setuptools: 38.2.3
Cython: None
numpy: 1.13.3
scipy: 0.19.1
xarray: None
IPython: 5.5.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0b10
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>