Hardened Parameter passing for Add User Stories and Sprints

The definition of a method in python is evaluated once so when instantiating a
new window, the default value for a parameter name=NewItem() is evaluated once
and future instantiations of that window will use that single instance of NewItem.

Apparently this is a python gotcha.

Fixes #31
/cc @sperovita
Allow Unpersisted items to have ID

When an item is unpersisted it has None for an id value. When stringfied this
should produce a consistent result that is not an error.