Testing guide update: add use case of testing the PeepCode Ordr App

This PR adds a complete use case for testing a non-trivial Ember.js application, the PeepCode Order App from the "Fire up Ember.js" tutorial video. 

This testing of the Order app was created as a project by the Ember-SC meetup group. Now that there is a "Testing" - Integration section of the Ember.js website... This case study can serve as a practical guide to using the Ember Test package, and also encourage the practice of end-to-end testing in addition to unit testing (for anyone building applications with Ember). 

The Ember-SC group did receive permission from PeepCode to publish the "Ordr" application code from the tutorial video as long as links back to the https://peepcode.com/products/emberjs page.