implemented Order and Product model associations and join table
modified Order/Product association, created LineItem model
implemented LineItem table in Order show view
added methods to calculate LineItem price and Order total
Order add and edit modifications for LineItem (in progress)
added composite key to LineItem and updated rails
removed 'total' from Order add and edit, as it is now calculated
added LineItem controller and routing
implemented LineItem create
finished add LineItem, deleting Order deletes LineItems, deleting Product deletes LineItems