allow object classes array modification

Tentatively closes #55 

added a little magic in models.base for unobtrusively observing changes to Model.object_classes. If this array changes because any MutableSequence methods (https://docs.python.org/2/library/collections.html#collections-abstract-base-classes) then make an original copy to use when compiling sql filterstring, otherwise a search against still-to-be-changed objectclasses list raise a DoesNotExist

Thank you, ciao