ENH/API: Scalar values for Series.rename and NDFrame.rename_axis
closes https://github.com/pydata/pandas/issues/9494
closes https://github.com/pydata/pandas/issues/11965

Okay refactored to overload `NDFrame.rename` and `NDFrame.rename_axis`

New rules:
- Series.rename(scalar) or Series.rename(list-like) will alter `Series.name`
- NDFrame.rename(mapping) and NDFrame.rename(function) operate as before (alter labels)
- NDFrame.rename_axis(scaler) or NDFrame.rename_axis(list-like) will change the Index.name or MutliIndex.names
- NDFrame.rename_axis(mapping) and NDFrame.rename_axis(function) operate as before.

I don't like overloading the method like this, but `rename` and `rename_axis` are the right method names. It's _mostly_ backwards compatible, aside from people doing things like `try: series.rename(x): except TypeError: ...` and I don't know what our guarantees are around things like that.
