This pull requests adds the function to disable the price per category

HI,

I have added the function of disabling of the price field per category. Sorry for the few extra commits as I had overwritten some changes that where made earlier. 

Also I am not exactly sure if the code I added to item.form.class.php is exactly the best way to disable the price field on the item post and edit page. 

This is related to issue #200 

Jay