Enable JSONP callbacks on REST blueprints

Since Express 3.x you now have to [explicitly opt-in to JSONP support](https://github.com/visionmedia/express/wiki/Migrating-from-2.x-to-3.x) and the only way to trigger a JSONP callback is by using the function call [res.jsonp](http://expressjs.com/api.html#res.jsonp) i.e. you can no longer override res.send() or res.json() by triggering enable("jsonp callback")

I've added an option into the controller config to allow JSONP callbacks in the REST blueprints.
By default JSONP is disabled.