API: add inplace keyword to Series.order/sort to make them inverses (GH6859)
closes #6859

now to be even more confusing

`DataFrame.sort(inplace=False)` is the default (and returns a new Frame)

`Series.sort(inplace=True)`
`Seires.order(inplace=False)`

To fix this is REALLY breaking the API

maybe add `DataFrame.order(inplace=False)`

and FutureWarn that `DataFrame.sort(inplace=False)` should be using order?
