Allow :route_set option for #render_constraints_query

At the suggestion of @jcoyne, this allows us to pass a `route_set` option in the parameters hash which is then applied to the `url_for` method.  This is useful for applications like Sufia, which are themselves engines that are using Blacklight.

Also, the EngineCart option prevents Spring from starting up which was cause troubles when trying to run the continuous integration task multiple times.