drpm: add config options + docs, load updateinfo.xml
drpm: download delta packages if possible
get_package_target(): support the "donecb" callback
drpm: spawn delta rebuild jobs, report failures
drpm: display delta rebuilds as they finish
drpm: fall back to rpm download on drpm errors
test_drpm: override the /bin/applydeltarpm detection.

Makes sure test_drpm tests pass even without "deltarpm" package installed.
Require it anyway to make it easier for users to enable with deltarpm=1.
Installing a local pkg is okay. (RhBug#1056400)
doc: update the deltarpm section in cli_vs_yum a bit.