Should Series.diff() have an axis argument?
I suggest that `Series.diff()` method should have `axis` argument to align its signature with the signature of `DataFrame.diff()`. This way it'd be easier to write polymorphic functions that could take both series and dataframes.
```python
# Series
x.diff(periods=1)
# DataFrame
df.diff(periods=1, axis=0)
```

Many other `Series` methods already have signatures identical to those of dataframes (and obviously when calling with `axis=1` would raise `ValueError`), e.g.:
```python
# Series
x.shift(periods=1, freq=None, axis=0, fill_value=None)
# DataFrame
df.shift(periods=1, freq=None, axis=0, fill_value=None)

# Series
x.cumsum(axis=None, skipna=True, *args, **kwargs)
# DataFrame
df.cumsum(axis=None, skipna=True, *args, **kwargs)

# Series
x.add(other, level=None, fill_value=None, axis=0)
# DataFrame
df.add(other, axis='columns', level=None, fill_value=None)

# Here axis is not an explicit argument, but properly passed from **kwargs
# Series
x.pct_change(periods=1, fill_method='pad', limit=None, freq=None, **kwargs)
# DataFrame
df.pct_change(periods=1, fill_method='pad', limit=None, freq=None, **kwargs)
```