uc_ajax_cart calls _uc_ajax_cart_load_res() upon module inclusion instead of using hook_init()

_uc_ajax_cart_load_res() gets called as soon as the uc_ajax_cart.module file is included, which in come cases can occur before bootstrap has finished. This results in an PHP error about drupal_add_* being undefined.

A simple solution is to rename _uc_ajax_cart_load_res to uc_ajax_cart_init(). hook_init() is only ever run once during bootstrap, so that also eliminates the need for the static $loaded variable.