'Obtain the final access token and secret for the API.
:param auth_data:   URL-encoded authorization data as displayed at
the authorization url (obtained via
:py:meth:`get_authorize_url`) after signing in
:type auth_data:    unicode
:returns:           OAuth resource owner key and secret
:rtype:             (unicode, unicode) tuple'

def get_access_token(self, auth_data):

    self.api.parse_authorization_response(('http://beets.io/auth?' + auth_data))
    access_data = self.api.fetch_access_token(self._make_url('/identity/1/oauth/access-token'))
    return (access_data['oauth_token'], access_data['oauth_token_secret'])
