remove: conf.keepcache option.

No real use.
doc: api: Base.reset()
doc: api: logging.
doc: api: initiali version of the plugin interface.
get_package_target(): don't use yum compatibility attribute
get_package_target(): support the "donecb" callback
drpm: add config options + docs, load updateinfo.xml
drpm: download delta packages if possible
drpm: spawn delta rebuild jobs, report failures