Added tests that include the pure python sets.Set().  Only the binary or/and/sub/xor methods are tested.   

The comparison operators were designed to only interact with their own kind.  A comment from Tim Peters explains the decision raise a TypeError instead of returning NotImplemented (it has unfortunate interactions with cmp()).  At any rate, nothing good would come from changing that design decision now, so I'm leaving it alone to fade peacefully into oblivion.