Local storage

I've added an option to save your done log locally in a json file (`~/.idid`). 

I've also refactored some of the app to create separate `Task` and `Delivery` classes and changed some methods from instance methods to class methods. This leads to a cleaner API, better testability and better separation of concerns.

I haven't added any tests yet, I will add them in this pull if you like the way I'm going here ;-).