make errata and updates (delta) json files dumped in rpmkit.updateinfo.cli
fix a bug that rk-updateinfo wrongly count sum of updates (actuall it was sum of errata)
implement basic list result caching mechanism in rpmkit.updateinfo.base.Base as these computation may become relatively heavy sometimes
add accesser (getter) for rpmkit.updateinfo.base.Base._packages
implement *_impl methods instead of interfaces in rpmkit.updateinfo.dnfbase.Base
process cases if (DNF) base.check_updates returns Hawkey query(?) object instead of list
updated Japanese translation
implemented basic dnf based function to list errata and add corresponding test cases