- Factor MailGW message parsing into a separate class, thanks to John

- Factor MailGW message parsing into a separate class, thanks to John
  Kristensen who did the major work in issue2550576 -- I wouldn't
  have attempted it without this. Fixes issue2550576. (Ralf)
- Now if the -C option to roundup-mailgw specifies "issue" this refers
  to an issue-like class. The real class is determined from the
  configured default class, or the -c option to the mailgw, or the class
  resulting from mail subject parsing. We also accept multiple -S
  options for the same class now. (Ralf)
- Add regression test for message-id generation if message id is
  missing in a message
- Add regression tests for Option parsing (-S and -C options)

git-svn-id: http://svn.roundup-tracker.org/svnroot/roundup/roundup/trunk@4577 57a73879-2fb5-44c3-a270-3262357dd7e2