retrieves all versions of an object, and stores them at self.versions

		Args:
			None

		Returns:
			None: appends instances

def get_versions(self):

		'''
		retrieves all versions of an object, and stores them at self.versions

		Args:
			None

		Returns:
			None: appends instances
		'''

		# get all versions
		versions_response = self.repo.api.http_request('GET', '%s/fcr:versions' % self.uri)

		# parse response
		versions_graph = self.repo.api.parse_rdf_payload(versions_response.content, versions_response.headers)

		# loop through fedora.hasVersion
		for version_uri in versions_graph.objects(self.uri, self.rdf.prefixes.fedora.hasVersion):

			# get label
			version_label = versions_graph.value(version_uri, self.rdf.prefixes.fedora.hasVersionLabel, None).toPython()

			# affix version
			self._affix_version(version_uri, version_label)