Import book from goodreads

Import a book from www.goodreads.com given a ISBN.
Added goodreads and webmock gems to the project
It imports:
* ISBN
* Title
* Authors
* Cover image
* Number of pages
* Descripton
Merge pull request #45 from groupbuddies/jj-import-from-goodreads

Import book from goodreads
Extracted responsibilities from book controller
Extend loan period
Show extend button only when extendable

* Only shows one week before the ending of the loan
* Can only be extended once
Notifies user that she can only renew once

* Show the expiration date for a loan on my shelf