Make rendr app mountable under Express 4

This simply makes it possible to mount a Rendr app inside an Express 4 app. The main issue was dealing with the newly-formatted `req.params` which are created by the hosting Express app at the newer version.

To mount a Rendr app in Express 4 you also need to supply an `errorHandler` in the options to `createServer` (otherwise Rendr falls back on `express.errorHandler()` which no longer exists), and `app.use(server.handle)` instead of `app.use(server)`.