* Added new attributes scopes and token_info_uri to OAuth2Credentials.
* Added method OAuth2Credentials.has_scopes to test which scopes the credentials have.
* OAuth2WebServerFlow now passes in the authorized scopes when constructing OAuth2Credentials.
* Added method Oauth2Credentials.refresh_scopes, which gets the canonical list of scopes from the OAuth2 tokeninfo service.
* Added new utility function string_to_scopes, the inverse of scopes_to_string.
This will fix #228 when merged.