Remove EmptyCart, use unsaved Cart instead.
Call cart.save() in add_item() if necessary.
Merge remote-tracking branch 'upstream/master' into remove_emptycart
Bind cart to session in recalculate_cart
Revert changes to cart view
Remove unused imports
Added string format method. #218
Merge
Added Kulbir Singh to AUTHORS.
Latest Django patch releases in travis config.