Enhanced lookup_for scope generation

Changes:

1. The scope is n-arity; there's no need for both `with_city` and
   `with_cities`. The plural form is no longer defined.
2. You can explicitly name the scope whatever you want.
3. `lookup_for` now accepts an `inverse_scope` option, as well, to
   generate `without_city` in addition to `with_city`.
Merge pull request #19 from pd/scoping

lookup_for scopes: n-arity; name the scope what you want; inverse scoping.