axis keyword of Series.ptp() does not work with 'index' as argument
According to the [API](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.ptp.html?highlight=ptp#pandas.Series.ptp), the `ptp` method of Series should be able to handle 'index' as an argument for the `axis` keyword. But when I try the following code:

```
x = pd.Series(range(15,20), index=range(5,10))
x.ptp(axis='index')
```

I get an error: "TypeError: an integer is required for the axis"

I'm using Pandas 0.17.0
