Added classic Mac OS support.

Should also work for OS X, though TickCount, which MacOS.GetTicks uses,
has been deprecated since v10.8. I'll test it later.

The MacOS module was removed in Python 3, but I'm not aware of any
versions of that being available for OS 9 and earlier anyway.
Document _uptime_mac.
Made _uptime_minix a separate function.

MINIX deserves better than to stand forever in Linux's shadow.
Version bump.