* should support system model
  - don't use conary API directly in conaryBackend.py
  - move self.xmlcache into conarypk, so it won't come in the way of system
    model support.
  - implement search (pending upcoming features of conary)
* fix use of post_process and implement _pkg_compare in conaryFilter.py
  - http://lists.freedesktop.org/archives/packagekit/2011-March/006245.html
* decide what to do with package components
* conarypk.py: move _classic_build_update_job()/_classic_get_package_update()/_classic_get_updateall_job()
  out of the ConaryPk and change them into functions, as the _model* functions.
