- fix serializer plugin interface to be able to deal with unknown parts

  when deserializing
- add temporary workaround for missing multipart support in
  ItemAppendJob until we get the real stuff from Robert's branch
- fix error handling in XesamManager
- let the maildir resource store all headers by default, which should
  make threading work even without relying on side-effects of the strigi
  feeder agent

svn path=/trunk/KDE/kdepim/akonadi/libakonadi/; revision=701767