Changes in integration with Wix: Added the "description" and "avail" parameters to the "...

Changes in integration with Wix:

• Added the "description" and "avail" parameters to the "category.add" and "category.update" methods
• Added field "create_at" to the "product.list" and "product.info" methods
• Added the "created_from" and "created_to" parameters to the "product.list" and "product.count" methods