api: added ucd and ucd_input

This should be IMO part of API in response to recent dnf-core-plugins bugs. Then I could add to plugins documentation how to use it on strings read outside dnf. From brief look I have seen `raw_input` (in copr plugin) and and `str` calls that should be replaced with `ucd`. I would also make compulsory  line `from __future__ import unicode_literals` to each plugin.