Pass locals into custom response functions

Previously, it was a pain to respond to a request with JSON while using `respond`. The controller object is cyclical, so passing it into `JSON.stringify` fails. Furthermore, that doesn't filter underscores.  

So, this simply passes properly-filtered locals into custom response functions. It could be used for all sorts of things that accept plain ol' JSON, but here's a simple example of responding with either HTML or JSON:

    pagesController.main = function() {
      var self = this;
      this.title = 'Locomotive';
      this.respond({
        html: true,
        json: function(locals) { self.res.json(locals); } // new magic
      })
    };

I've only tested manually, but I'd be happy to write up some tests!