Series.ptp function does not correctly use axis='index' argument
When using [Series.ptp()](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.ptp.html?highlight=ptp#pandas.Series.ptp) with `axis='index'` as an argument, I would expect the function to return max(index) - min(index). Instead, it appears as if the operation is performed on the data held in the Series (as if the `axis='index'` argument were not provided at all).

```
x = pd.Series(np.linspace(0,5,6)*2, index=np.linspace(0,5,6))
print x.ptp() #expect 10, get 10
print x.ptp(axis=0) #expect 10, get 10
print x.ptp(axis='index') #expect 5, get 10
print x.index.max()-x.index.min() #expect 5, get 5
```

I'm using Pandas 0.17.1.
