Zero value

I've added support for querying with the zero value to `without_attr`, `with_any_attr`, `with_exact_attr`, and `attr?`.

Some concerns from @spemmons:

> originally, before the introduction of the :zero_value idea, "some_bitmask?(*values)" was intended to return true if ALL the symbols in the values list are bits set in the bitmask; since :zero_value isn't strictly a "bit" it seems odd to test for it being "set"; if we DO allow it to be passed in the values list, perhaps it should only be allowed as a single value AND the bitmask be set to true... thoughts?

Thanks for your feedback @spemmons!  I agree that it's a bit odd to test for the zero case since there isn't actually a zero bit.  However, it definitely comes in handy and is a nice alternative to `mask.blank?`.  Also, I don't think that any extra testing is required for passing the zero value.  If you pass the zero value with other values you're guaranteed to get `false`, which seems correct to me (i.e. `attribute?(:value, :zero)` always returns `false`).

As for the other methods, it's very handy to have a scope that will handle the zero value correctly, especially since it encompasses `nil` behavior as well.  Let me know what you think!  :)