Register a runner on the backend and report it's active: - runner_id - String, unique identifier for runner. - active_at - ISO String timestamp. Optional. If none is given, current time - callback - f(err)
function registerRunner(runner_id, active_at, callback) {
        if (typeof (active_at) === 'function') {
            callback = active_at;
            active_at = new Date();
        }
        if (typeof (active_at) === 'string') {
            active_at = new Date(active_at);
        }
        runners[runner_id] = {
            runner_id: runner_id,
            active_at: active_at,
            idle: false
        };
        return callback(null);
    }