Display cart in the sidebar.

I really don't understand some of these changes yet. I'm going to finish
the chapter, then try it again tomorrow and see if it makes more sense.
Redirect new cart items to store, not cart.
Merge branch 'cart_to_sidebar' into develop
Ajax v1. Though I don't really get it all.
Merge branch 'ajax' into develop
Highlight changed cart item in the sidebar.
Merge branch 'highlight' into develop
Hide empty shopping cart.

This doesn't work perfectly. An empty cart is hidden, but when you add
the first item, it reveals an empty cart. A 2nd item added does show two
items. There's also a window flash between some actions.

But right now, I'm not worried about Ajax. I just want to keep moving
forward to the stuff I'll need to use sooner.
Merge branch 'hide_empty_cart' into develop
Click on entry image to add to cart.
Merge branch 'clickable_images' into develop
Test new ajax features.
Merge branch 'testing_ajax' into develop