doc: tweak the 'proxy' documentation in conf_ref.rst
doc: fix a semantic error in cli_vs_yum's skip_if_unavailable doc.

Thanks Kamil Paral for finding this!
doc: using --setopt from CLI. (RhBug:1044981)
doc: cli_vs_yum: no 'Processing dependency' lines. (RhBug:1044999)
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
drpm: display delta rebuilds as they finish