Waiver Signed Required Checkbox added + Shopping Cart

- Added a checkbox (Waiver Signed?) With validation
- Added "Add To Cart" button at bottom of form
- New method in ctlRentalForm  addToCart() --> When customer hits add to
cart, rental gets put into cart and redirects to index() displaying
shopping cart.

TO DO: In comments on shoppingCart.php view
1) Make shopping cart fully functional and display all desired fields
(Product ID, Duration etc)
2) Make shopping cart editable (delete, modify items?)
3) Carry out add to database form shopping cart (will have to modify
earlier created Add Rental method)