Need to replace the "dom-to-model" bits in the cart notifications JS

[jtsternberg]

Currently (now that #2295 was merged), the JS cart modal stuff is depending on some brittle html bits in the dom in order to add an item to the cart and display it properly. We need to update this to output actual data models in the dom for every product shown on the screen.

[jtsternberg]

Or, probably more ideally, using the REST API (WPSC_REST_Products_Controller) to do an ajax fetch when an item is added to the cart.