Bumping to 0.9.24; going to add a new feature momentarily.
Improved usage output (e.g., that of "tuxpaint --help"); broke
the options into groups that match the tabs in Tux Paint Config.,
added some missing options.
Documenting Mark Kim's recent changes:
* Less CPU-hungry progress bar (during Stamp loading)
* Verbose debugging options
* (Also, mentioned debugging _at all_, in INSTALL.txt)
Bumping version to 0.9.23a, to match the test build he's working
on for macOS.
