Add initialize function for server setup

I often find myself wanting to interact with the express server and set it up a certain way before making requests to it. For example, if you're testing a global middleware and want to use some jade templating:

```js
var fixedServer = FixedServer.fromFile('./fixtures/fixed-server', {
  port: 1337,
  initialize: function (app) {
    app.engine('jade', jade.__express);
    app.set('view engine', 'jade');
    app.use(myGlobalMiddleware());
  }
});
```
And if you'd like to set your server up on a per-test basis, you could do something like (with the addition of #5:

```js
fixedServer.createServer(['GET 200 /']).setup(mySetupFunc).run();
```

This might not be the best approach, but something along these lines would be very useful!