Add a class that represents an OAuth 2 token response

[justincy]

The GEDCOM X spec recommends OAuth 2 be used for authentication but does not demand it. Thus we'll add it as a FamilySearch extension.

From the OAuth 2 spec, here's the format for a successful response and an error response.

This is only relevant for the token endpoint.

[justincy]

This class doesn't need to support XML.