Use a setter for renderer

It would be helpful to store the renderer in the object, so one can define it when expanding the object and not have to do it on every instance.

eg. add a function like:

        setRenderer: function(renderer) {
          if (typeof renderer === "string") {
            this.renderer = this.renderers[renderer];
          } else if (typeof renderer === "object") {
            this.renderer = renderer;
          } else {
            this.renderer = this.renderers.underscore;
          }
        },

and do

          this.setRenderer(opts.renderer || this.renderer);

in the initialize-function.

Atle