Function request_token 

pub fn request_token<C, S, B>(
    client_credentials: &Credentials<C>,
    callback: &str,
    x_auth_access_type: Option<AuthAccessType>,
    client: &mut S,
) -> AuthFuture<Credentials<Box<str>>, S::Future> 
where
    C: AsRef<str>,
    S: HttpService<B>,
    B: Default,