accomodate for new iterator in DOMTokenListPrototype

Starting with Firefox 28, DOMTokenListPrototype defines an iterator
function called '@@iterator'. That breaks the assumption that all
child objects prefixed with '@' are designed attributes.

For now, we'll just skip operations on child objects prefixed with
"@@" (i.e. having a second '@' in string position #1).
Merge pull request #2 from hwhw/develop

accomodate for new iterator in DOMTokenListPrototype