Add update object.
Add updatelist object.
Add updateref object.
Add updatereflist object.
Add hy_package_get_updates method.
tests: add tests for update object.
tests: add tests for updateref object.
py: add _hawkey.UpdateRef type.
py: add _hawkey.Update type.
py: add load_updateinfo kwarg to _hawkey.Sack.load_yum_repo.
py: add _hawkey.Package.updates getter.