added shuffle method to DataFrame
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I have added a shuffle method to pd.DataFrame, as writing df.sample(frac=1) is not intuitive. The method just calls df.sample with 1 line of code.
To test it, I have added a test, here is the code:
```# flake8: noqa

"""
Public testing utility functions.
"""
import pandas as pd
from pandas.util.testing import (
    assert_frame_equal, assert_index_equal, assert_series_equal)


df = pd.DataFrame([[1, 2], [3, 4], [5, 6]], columns=['a', 'b'])
shuffled = df.shuffle(random_state=1)
expected = pd.DataFrame([[1, 2], [5, 6], [3, 4]], columns=['a', 'b'])
expected.set_index(pd.Series([0, 2, 1]), inplace=True)
assert_frame_equal(shuffled, expected)
```
This PR doesn't resolve any issue, I just thought a shuffle method would make a nice addition to pandas.
Also, I did not understand what you mean by 'whatsnew entry', could you please clarify?