Local storage

Fixes #4 

## Additions
I've added an option to save your done log locally in a json file (`~/.idid`). As a bonus, I've also added the `--list` option, to show which tasks you logged for a given date. You can also use `-l` instead.

    $ idid --list 2012-12-25
    Log for 25 December 2012
    * Jingled bells
    * Ate a turkey
    
    $ idid --list
    Log for <today>
    * Created pull request for idid gem

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 ;-).