Preparation to store last ID in xml file

Preparation for #231 
XML is going to split into two nodes, `<tasks>` and `<data>`. This PR implements the splitting, and getting of those nodes, but have not supported modification to the `<data>` node yet.
Most probably going to refactor `getTasksNode` and `getDataNode` to another class implementing `IOperation`.
Going to overload `Post`, `Put`, and `Erase` so that if it receives `wstring` instead of `TaskId`, it will go to the `<data>` node.
Please advise if the above is fine.