Use `None` as the default for "feature scaling" parameter

[desilinguist]

Do it in two stages, i.e. two releases.

1. The first shows a deprecation warning if "none" is specified.
2. Using "none" raises an error.

[tamarl08]

@desilinguist ConfigParser supports only string values. How about we make it an empty string?

[desilinguist]

Yeah that works too.