Adds a with_raw_uuids scope to the uuidify concern

Added a scope like this on a uuidified model for my specific use case, so that I could grab sets of results along with their uuids in a single query (which is not possible with an ActiveRecord `includes`, since the concern does not establish a normal active record relation between the uuidified model and the `Uuidify::Uuid` model).

Not sure if this is useful enough to merge back into the gem, but I thought I'd submit a PR and let you decide.