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)
The image target here should be a little rotating selector with a few lines of JS so that visitors also have a chance to see the other books. See how we rotate the nbviewer screenshots for an example you can crib and reuse.  The easiest way to allocate the links so that, say, 1/4 is the old book and 3/4 the new, is to make the list of images 4 long and put the old one only in one slot, with the new one in 3. No need to mess with probabilities in the code itself.