Handle unhandled requests

Issue #120 happens when a request is captured by Primus and it is not correctly handled by the underlying realtime framework.
`Socket.IO` and `Engine.IO` correctly handle this.
This patch adds:
* a listener for normal requests in the `websockets` transformer which returns a `400` error status code
* a slightly modified prefix for the `sockjs` transformer to make it work like `Socket.IO`/`Engine.IO`
* a `next` handler which emits the `previous::request` event on the transformer when the `browserchannel` transformer returns `next()`