Update xmlgenerator.php

After some trouble with XML and Skroutz suggestions about the structure of XML I have made some changes. Below are the changes as the skroutz suggest me...
1. Added manufacturer to the main title
2. Added the variable $vat_value to hold the VAT because product_price is without vat and math exp to calculate it. 
3. Added product avaliability after stock value check
4. Changed the exp when loop and add the products to the array so to check and export the products with unique ID and/or same category. That because of double records of both parent and sub categories.

Please feel free to corect me for any mistakes I was made.
Thank you.