widgets: don't subclass widgets.SelectMultiple
widgets: don't subclass widgets.SelectMultiple

So that the infamous "Press Control or Command" help text is not
displayed on django 1.6+.

Fixes #5

Ajeetlakhani/django-mptt-autocomplete,xrmx/django-fancytree,Ajeetlakhani/django-mptt-autocomplete,xrmx/django-fancytree,xrmx/django-fancytree,Ajeetlakhani/django-mptt-autocomplete