Allow alternative names for the OAuth access token type name

This ensures backward-compatibility with earlier version of the
"Bearer Token Usage" draft
(http://tools.ietf.org/html/draft-ietf-oauth-v2-bearer-23#appendix-B) when the
OAuth access token type name was named "Oauth2" as well as with
earlier / "exotic" implementation of the "Bearer Token Usage draft" where the
OAuth access token type name could be "Oauth" or "Token".