Support for OAuth scopes, default client

- oauth_default_scope stores a single row, which will be used as the
default scope for all clients. This should expand to allow per-client
support.
- oauth_supported_scopes stores one row per client_id per scope
- oauth_default_client stores a single row, which can be used to define
the default client for any purposes (NOTE: this table has not methods
explicitly used by library)