basic skeleton test harness for testing debutils.Repo.Release
added test parameter to download and parse Debian/sid's Release file
added parameter to test parsing against Debian/sid's Release.gpg signature file
added missing optional field "Valid-Until"
can import debutils.Repo.Release instead of debutils.Repo.release.Release
started fleshing out the API for debutils.Repo.Release. PyGPGME was determined to not be a good fit, unfortunately. Most of the other Python GPG wrappers seem to basically do the same thing - shell out to the gpg binary for all tasks, which is not acceptable for this project.