Added 'books' page

As discussed on the [mailing list](http://mail.scipy.org/pipermail/ipython-dev/2014-August/014747.html), this PR:

* adds a new `books.html` page listing two IPython-focused books in reverse chronological order
* adds an announcement in `news.html` about the release of the IPython Cookbook
* adds an announcement in `index.html` about the release of the IPython Cookbook
* modifies the book cover image in the sidebar: it alternates between the older minibook (1/3) and the newer cookbook (2/3) (the current JavaScript implementation is probably not ideal, awaiting suggestions)
I think that by nbviewer Fernando was refering to the sidebar of http://ipython.org/ that links to nbviewer.