Downloads Project Gutenberg RDF catalog.
	Yields:
		xml.etree.ElementTree.Element: An etext meta-data definition.

def getrdfdata():
	"""Downloads Project Gutenberg RDF catalog.
	Yields:
		xml.etree.ElementTree.Element: An etext meta-data definition.
	"""
	if not os.path.exists(RDFFILES):
		_, _ = urllib.urlretrieve(RDFURL, RDFFILES)
	with tarfile.open(RDFFILES) as archive:
		for tarinfo in archive:
			yield ElementTree.parse(archive.extractfile(tarinfo))