fix: gracefully handle the exception when cannot create the locks directory. (RhBug:1036147)
tests: add repository with delta rpms
tests: test drpm lookup, test basic rpm downloading
api: Extract repo ID validation to a separate function (RhBug:1018284)
doc: clarify repo_id_invalid() slightly.
doc: document the "proxy" option.
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.
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)