Commit before starting chapter 10 task E
Added migration:  rails generate migration add_quantity_to_line_items quantity:integer
Show quantity for multiple items rather than repeating items
Fixed missing end in cart.rb
Added up and down methods to combine multiple items into a quantity>1 line item in all carts
Handle errors gracefully when presented invalid cart ID
Remove cart_id as allowable parameter
Fix test to prevent error relating to cart_id parameter
Create 'Empty Cart' button, corresponding test and controller
Remove line item created confirmation message
Nicer-looking shopping cart