ActiveRecord::Relation subquery support

This is twofold.

ActiveRecord::Relation where clauses hash syntax accepts an ActiveRecord::Relation as a value, but it maps the relation to ids when it could just us a subquery. By adding that support we can easily add support for subqueries in general.

I've attached a patch with tests.

I've corresponded with Aaron Patterson a bit and he told me to assign it to him, so I'm doing so.