created item can share objects with parent item

If child & parent dont have the same key defining an object every object
in parent are shared with child.
Else object are inherited from parent and merged.

Example:
Item.define('a', {options: {name: 'ok'}};
Item.extend('a', 'b');
Item('a.b').options == Item('a').options -> true

Item.extend('a', 'c', {options: {name: 'foo'}});
Item('a.c').options == Item('a').options -> false

v