Going through their license FAQ, I found this,
Do I have to credit the author of the item in my end product?
"No, it's not mandatory to give the author credit. But we do suggest that if your end product has credits as part of its design, please credit the author. Also, as the author retains ownership of the item, you shouldn't claim copyright in the item."
How strong is the word shouldn't here? Does it mean that I cannot use the following information at the footer,
© Copyright 2013.
All rights reserved.
or again, it is ethically implied? If the first condition applies, can I refer to the rights reserved concerning the content of the website excluding the design?
Again, thank you.