support for IS NULL predicate

hi

it seems that support for NSPredicate "== nil" (resulting in IS NULL) was missing.
I've added some solution to this problem, although it does not play nice with the current approach to operator (operators NSDIctionary). So basically the "=" operator needs to be replaced with "IS" for IS NULL to work. I've just accomplished it by a check if there is an rightOperand but no rightBindings. In this case we might (?) assume that it's a "IS NULL" case and provide a special version of query.
What do you think?

Best regards
Rafal