A simple validator. Can be used to check that all members expected are
present.
To use it, provide a configspec with all your members in (the value
given will be ignored). Pass an instance of ``SimpleVal`` to the
``validate`` method of your ``ConfigObj``. ``validate`` will return
``True`` if all members are present, or a dictionary with True/False
meaning present/missing. (Whole missing sections will be replaced with
``False``)