Remove implicit "neutral values" in __init__
When an item is created or loaded from the database, fields for which there are no known values are explicitly set to a neutral value like "0", "<empty string>" or datetime.now().
This is redundant with default types constructors. It is also a bad idea to rely on these values when instanciating a new Item. It will thus be removed.
If you need default values to ease objects creation, just use the default dict in your model or the init method