rename: dnf.persistor.Persistor -> RepoPersistor.

It is unfortunately repo specific since it takes cachedir.
logging: log plugin import tracebacks on the subdebug level.

Else they can mess up a '-v' CLI big time.
groups: add dnf.persistor.GroupPersistor(), to store the installed groups.
groups: add support for marking/unmarking groups.
groups: add Base.group_remove().

Used from CLI.
groups: add installing groups the object marking style.
Allow installing optional packages from a group. (RhBug:1067136)
groups: recognize 'mark install' instead of 'mark-install'.

Yum documents only the former, as it should.
api: add Base.group_install() with exclude capability.

Anaconda needs to be able to do the excluding. Also add
Base.group_remove().
doc: document the group marking operations.
api: deprecate Base.select_group()
api: drop items deprecated since 0.4.9 or earlier.

This is in accordance with our Deprecation Policy.