Book module

Steps:
- Create a API module in the Node.js app (751e40f)
  * Define the model and export it ([`book.model`](https://github.com/aliou/tsundoku/blob/books/app/api/books/book.model.js))
  * Define the controller method needed ([`book.controller`](https://github.com/aliou/tsundoku/blob/books/app/api/books/book.controller.js))
  * Finally, create the router to serve the controller method ([`router`](https://github.com/aliou/tsundoku/blob/books/app/api/books/index.js))
- Create an angular module:
  * Create the provider route ([`app.js`](https://github.com/aliou/tsundoku/blob/books/public/js/app.js#L7-L10))
  * Create the service getting the books ([`book.service`](https://github.com/aliou/tsundoku/blob/3299b27311d30aaf178ec36163083673979abd96/public/js/books/book.service.js))
  * Get the books in the controller ([`book-list.controller`](https://github.com/aliou/tsundoku/blob/books/public/js/books/book-list.controller.js))
  * Create the template to show the books ([`book-list`](https://github.com/aliou/tsundoku/blob/books/public/js/books/book-list.html))
'exports' is not defined.