Add fields "taskid" and "created_at" to Todo items

    * The class 'List' is now obsolete and will be deleted soon
    * The class 'Task' substituites the class 'List'
    * The new fields are saved together with each item in the list.yml file
    * The list command shows a slightly more user friendly output, containing all the fields per each todo item
    * Adapt lib/{cli.rb, store.rb} to work with the Task instead of the List objects