Complete Task 6: Build a Cart

* Add products->get_many() method
* Add cart helper class
* Update product page to include product id in add to cart form
* Fix bug for detecting if there are products in the session
* Add missing parens in Products->get_many()
* Fix bug in querying for many products
* Add some convenience functions to cart items
* Fix variable name
* Fix Product->get_many for empty request
* Cart view uses session data
* Change link href in cart page
* Remove element js handles new product id data format
* Add remove from cart php page
* Annotate cart html with line total information
* Update totals on cart page after remove
* Remove old remove from cart file