lots of changes to the model, e.g.:
- use iso-format of date in db.xml again
- added/fixed comments
- added notification of observers
- added some methods to get a single item and the index of an item
added a TableModel, that can be used for a JTable to display all
ToDoItems

The order of the columns is defined in the ToDoItem-class via
annotations. The Label of the columns is just hardcoded in the
TableModle atm and should be transfered to a file when we work on the
internationalization.