tests: MockBase() should load real repos, too.
drpm: add config options + docs, load updateinfo.xml
drpm: download delta packages if possible
tests: test rpm and drpm downloads from repo with deltas
get_package_target(): support the "donecb" callback
drpm: spawn delta rebuild jobs, report failures
drpm: fall back to rpm download on drpm errors
tests: test drpm to rpm failover
Hide the progress bar on failfast abort
fix some uses of str() in unicode context. (RhBug#1044502)

Note: if the object implements both __unicode__() and __str__()
properly (dnf.exception.Error does), we don't have to use any
conversions and can append/interpolate the object as is.