Add attribute id on Book instance object

Add unique strings given to each volume that Google Books knows about on Book instance object.
https://developers.google.com/books/docs/v1/using?csw=1#ids

This unique string id (aka: Volume IDs) could be useful to retrieve a specific book. This id is available in books' url on google play store, so when parsing urls, we could retrieve that book, but Google Books API does not care about the case (lowercase/uppercase), example:

https://www.googleapis.com/books/v1/volumes?q=rokQngEACAAJ

You get 14 items with the same id, but with different cases.

So I added Book#id with attr_reader to be able to make some detection like that:

id = "rokQngEACAAJ"
response = GoogleBooks::API.search(id)
book = response.detect { |book| book.id == id }

I have also updated a little bit the specs to fit with the present API data, and all specs pass!