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)
Yeah, it would be cleaner, but I wanted the image to show up even with no JavaScript.

I'm not sure I understand how it works on nbviewer: the screenshots seem to be the same when I refresh on http://nbviewer.ipython.org/. What I could do maybe is to make my implementation more symmetric, by setting the new cookbook value dynamically rather than statically?