WorkflowId and Last Received Event

Adding in the workflowid and the ability to retrieve the last event in the history.

Tests include calling both features on an object with many events, and calling it on an object with no events.
I thought of one more thing. Does AWS index the events starting at 0 or 1? If 0, I think this function may fail if the only event has ID 0 since `null < 0` is `false` in PHP. You'd either need use `<=` or check if `$maxId` is null.