get $not working on mongo via $nor

 * modular-odm's "not" wasn't working on MongoDB, because their $not op
   cannot modify an equivalence like {a: "foo"}, and no $eq op is
   available.  Thankfully, they provide $nor and boolean logic dictates
   that (A $nor A) == ($not A).

 * remove comment documenting this failure from test_logical_operators