Add an advanced example to express-state

This pull request adds a more advanced example to `express-state`. 

## Overview
The example itself is an Express app similar to the basic example. After thinking about it for a while, I decided it would be useful to break up the example into smaller examples. The index page of the app points to 2 smaller examples, which are more realistic and full-fledged than the existing basic example. 

## Use-cases that I cover
The two use-cases that I cover through the example are:

* Getting some data on the server and exposing it to the client, where it is used to create a chart using JavaScript (I used d3.js for this)
* Exposing some config object to the client and using it to get something dynamic (I get recent photos from Flickr by exposing a Flickr API key)

I'd be interested in getting feedback on this stuff. Are there any other use-cases which you think would be valuable to include? What do you think of the way I structured the project directory? Let me know what you think!