Added support for overriding the redirectUri at runtime.

Now you can use a dynamic hostname for the redirectUri, like so:
```javascript
  app.get('/auth/google/callback', function(req, res) {
    var google_redirect_uri = 'http://' + req.get('Host') +
    '/auth/google/callback';
    passport.authenticate('google', { redirectUri:
      google_redirect_uri})(req, res, function(){
          res.redirect('/');
        });
    });
```