refactor: Raise/catch proper exceptions in/by package-marking methods.
remove: Do not give package-marking exceptions translated messages.
refactor: Do not give package-marking exceptions long messages.
tests: Make tests independent of locale settings.
Support iteration over packages installed by the user.
Support printing of package names.
Add "history userinstalled" command (RhBug:884615)
po.localPkg(): return the right localpath. (RhBug:1034607)

- repo.pkgdir should always return a writable dir.
- smarter repo.local, so we don't have to parse baseurl again.
- fix po.localPkg() to work with local repositories.