Initial start on an extended example

Jumping from a simple example to the Zen of Angular was a bit jarring.  The story-line of the invoice can be changed or expanded to expound on additional aspects of Angular.
* Explain about the "currency" filter and show its code;
* Use ng-repeat and expand on models to allow a set of quantity/cost pairs;
* connect a "Pay Invoice" button to mock payment form - to get across routing and