Nan & Bob have asked us to spice up the home page of their new site with a photo background - but they think the text is hard to read when scrolling, so the photo background needs to go away when the user is scrolling the longish content.
See Chris Coyier's excellent article (from which we fashioned the CSS for the bookstore background image in this exercise) for more info on stretching background images.
bookstorebg to the home page content
div allow us to style it, initially, with the CSS class already written. We add id
homecontent to the home page content, too, so that we can reference the element in the event handler.
scrollstop - to add and remove the class from the div in response to user scrolling.