uc_discounts_alt should use Drupal behaviors to insert the discount information on the cart page instead of inline JavaScript

uc_discounts_alt uses inline code to insert the discount information before the buttons on the cart page, which means that if the page is modified dynamically the discount information is lost (because the inline code only runs on page load). Attached patch uses Drupal behaviors to insert the discount information, which will run on page load as well as every time the page DOM is modified. This fixes a problem with uc_ajax_cart (see #1437820: uc_ajax_cart should use attachBehaviors to notify other modules of cart reloads).