compare.py: enable use of different sorting key functions for file names in Compare class.

Updated the Compare class to enable a sorting key function to be explicitly specified, and
implemented a new SortKeys class providing methods for a couple of sorting key functions
(including 'natural order sort' used by Windows Explorer).
go_compare.py: add option in GUI to use 'natural sort order' for filenames (emulating Windows Explorer).
compare.py: add --use-natural-sort command line option (to emulate Windows Explorer file ordering).
go_compare.py: give focus to the first directory selection line on GUI startup.
go_compare.py: automatically set the option to use natural sort order if running on Windows.
Updated README for new option on compare.py (--use-natural-sort).
Updated version to 0.0.4.