Allow updating the inventory quantity for a given book

[drborges]

Currently, the BooksController#update is stubbed out.

We need to integrate the update action with Shopify's API to allow users to update the quantity of a book product. At the moment, book updates should only affect its inventory quantity.

There's some discovery that needs to be done before we can implement this feature:

1. How will the inventory be structured in Shopify?
2. Will there be multiple locations?