Next & Previous Buttons on Product Page

This vqmod file doesn't edit any core file! On every product page you will see next and previous buttons, in the same category as the product. If you want this function to work perfectly, each product must be linked to a single category. Demo: http://demo.elisfashion.ro/ Tested on default theme on version 1.5.6. Please tell me if it works on any other versions or themes. Enjoy!