exposes the real engine.io server from the attach method (as it does in the main fork) so that `clients` and `clientsCount` can be exposed

Before submitting this PR I ran the tests and noticed the following (possibly unrelated) message on `Darwin dev04.local 13.0.0 Darwin Kernel Version 13.0.0: Thu Sep 19 22:22:27 PDT 2013; root:xnu-2422.1.72~6/RELEASE_X86_64 x86_64` Running node `v0.10.18` with npm `1.3.8`.

```
➜ engine.io-stream git:(master) ✗>npm test

> engine.io-stream@0.4.2 test /Users/paolo/workroot/git/hij1nx/engine.io-stream
> zuul -- test/index.js

  - all passed

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: getaddrinfo ENOTFOUND
    at errnoException (dns.js:37:11)
    at Object.onanswer [as oncomplete] (dns.js:124:16)
    at Socket.recon_local (/Users/paolo/workroot/git/hij1nx/engine.io-stream/node_modules/zuul/node_modules/localtunnel/client.js:76:23)
    at Socket.g (events.js:175:14)
    at Socket.EventEmitter.emit (events.js:92:17)
    at Object.afterConnect [as oncomplete] (net.js:883:10)
npm ERR! weird error 8
```