Fix CREDITS to reflect GitHub usernames as well as Subversion.
Update some references to Parrot, Perl, and/or Rakudo.
Add CREDITS (distilled from Parrot repository CREDITS file).
In creating this file, I tried to filter those entries that
were specific to Rakudo. If your entry was removed and you
want to remain in CREDITS, just send a patch and we'll put
you back in.