Breadcrumb bugfix when using the parameter 'startup_directory_method'

Bug: when using the parameter 'startup_directory_method' the breadcrumb (everything that come right after the "you are here: ") is always reduced to a link to the current directory. When a subfolder is entered, there is no way to navigate back to its parent.

Bugfix proposal: the method 'getStartupDirectory' expects the current at object where the reference field is defined (= self.at_obj) and not the current directory where the view is browsing to select objects to refer (= context). 

In the method 'getStartupDirectory' of widget.py. Line 55 It tries to do
method = getattr(aq_base(instance), self.startup_directory_method, False)
The getattr will always fail and return False since 'instance' is not the archetype object on which the method is defined.

I hope I'm clear in my explanation, else, ill just provide a concrete example.