Issue title: sessionStore deletion, server.request middleware call
 Issue body: So I've been trying to figure this out for a few hours, forgive my naivete.

When using connect's [MemoryStore](http://www.senchalabs.org/connect/middleware-session-memory.html), (yeh, yeh I should be using redis, this is educational only.) Where is the Session stored?

If you don't assign a variable to it... and allow Express to initialize the default MemoryStore

```
app.use(express.session({ secret: "keyboard cat" }));
```

How do you access the _MemoryStore_ ? Anything like, _server.routes.session.store_ ?

I know you can access it with _req.sessionStore_, but I'm trying to access it with only the _server_ variable. So my follow-up question is how do you simulate a request, or does Express have anyway to do "fake" requests? Like...

```
server.runFake(function(req, res){
req.blah  // Do some req/res stuff
console.log(res...)
});

// ...or...

console.log(server.request)  // Request object ?
console.log(server.response)  // Response object ?

```

---

Basically, must I have to do something like...

``` javascript
server = express.createServer({...})

server.get('/delete', function(req, res){
 req.sessionStore.clear()
});

require('superagent').get('http://localhost:3000/delete')
```

just to clear the sessions, with default _MemoryStore_, or is there a better way?

Thank You!

 Comments: 
 Comment 0: We had the same issue just today attempting to access the session from socket.io's `connection` event.

I poked around in the debugger and in the codebase to try and find where express was keeping the session stores but I could not find it.

We solved the issue by creating the store ourselves and passing it to express...

``` coffee
express = require 'express'

sessionStore = new express.session.MemoryStore

app = express.createServer()
app.configure ->
  app.use express.session store: sessionStore, secret: 'top secret'
```

This way you have the reference `sessionStore` to the session store.

Let us know if there's a reference from `app`...

 Comment 1: Maybe its not relevant anymore, but check out this line: https://github.com/senchalabs/connect/blob/master/lib/middleware/session.js#L235

You should be able to access it through `req.sessionStore`

 Comment 2: you both got an answer. however, you should avoid using memory store as no one should ever use it in production. use a real store or cookie sessions.

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