Add attribute id to Book object

Unique strings given to each volume that Google Books knows about.
Add specs for Book#id
Update subject title with updated API values

https://www.googleapis.com/books/v1/volumes?q=isbn:9781935182320
Remove useless specs for published_date

publishedDate will be always a string, and Book object
only cast to string, no need to have different specs
Remove unneeded specs for Book#publisher

publisher method is already tested in book specs
Merge pull request #2 from madwork/add-missing-unique-id

Add attribute id on Book instance object