AddTask adds a task with the given description to the datastore, returning the key of the newly created entity.

[START delete_entity] DeleteTask deletes the task with the given ID.

[START retrieve_entities] ListTasks returns all the tasks in ascending order of creation time.

[START update_entity] MarkDone marks the task done with the given ID.

[START format_results] PrintTasks prints the tasks to the given writer.

[START add_entity] Task is the model used to store tasks in the datastore.