Prepared for depreciation of natsort_key.

The natsort_key function was changed to _natsort_key, and a new
natsort_key function was created that is a wrapper for _natsort_key
but also will raise a DepreciationWarning via the warnings module.
Documented changes in README.

This includes changes that have not yet been made, but will be for the official
release.
Also uncommented all calls in profile code.
Added the as_path option to natsorted & co.

The as_path option allows the user to specify that the input strings
should be interpreted as file paths, and thus split on the path
separator.  It also splits on the file extensions.  This helps sorting
some OS-generated file names like "Folder (1)/" and "Folder/", where
"Folder/" should come first, but without as_path it would come last.