0.7.0

• Add GoogleCredentials.createDelegated() method to allow using domain-wide delegation with service accounts (#102 addresses #90): see https://developers.google.com/identity/protocols/OAuth2ServiceAccount#delegatingauthority

• Retry HTTP errors in ServiceAccountCredentials.refreshAccessToken() to avoid propagating failures (#100 addresses #91)

• Allow bypassing App Engine credential check using environment variable, to allow Application Default Credentials to detect GCE when running on GAE Flex (#103)