Fleshed out example so that cart / product functionality works in a live, runnable example

The current repo only shows, through code, how the example is implemented.  I went ahead and filled out the details so that users new to Rails can actually run the example, add items to the cart, and see everything working as it should.  Basic shopping cart functionality gems are quite hard to find, and this one seems to be the simplest; it deserves a working example.