Added option to use custom preprocess middleware.

Pass a middleware as ```preprocess``` option to ```PeerServer``` like so:
```javascript
var PeerServer = require('peer').PeerServer;
var server = new PeerServer({
  port: 9000,
  preprocess: function (req, res, next) {
    // Do something with the request here,
    // Set some headers,
    // Blah..

    return next();
  }
});
```