Important changes to simplify traitlets. Some aspect of traitlets were a bit too magical for us. Thus, we have simplified certain things to make it more straitforward: * Default values are always validated, but now, this is done when the HasTraitlets.__new__ is called. We used to do this the first time __get__ was called. * The klass argument of Type and Instance traitlets must be a class. Unlike enthought.traits, we are no longer accepting class names as str or instances. * The args and kw arguments to Instance.__init__ are now handled in better manner, but that is quite different from enthought.traits. * More tests of edge cases.