Move kraken to app.locals, where it should have been originally.

It was silly to add kraken directly to app. This is the first step in correcting that mistake. Later a warning can be put in the getter and after a time the app.kraken property can be completely removed.

Also, intentionally making property non-enumerable to prevent [copying of `app.locals.kraken` to `res.locals`](https://github.com/visionmedia/express/blob/6775658ed5cbac13f2d24c89e23ed967ad6a66ba/lib/application.js#L483).