[Spambayes] Danger, Will Robinson - BAYESCUSTOMIZE change!

[Skip Montanaro]

I just checked in the os.pathsep fix to Options.py.  Note that this will
change all uses of that environment variable.  No longer will you use a
space to separate multiple filenames.  Unix-y systems will use ":".  Windows
will use ";", Mac pre-OSX will use "\n", and RiscOS will use ",".

I also added a note to README.txt.  I didn't see any other necessary changes
in the distribution code.

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