Split out a do_set_attribute method.
Move SetAttribute onto JS<Element>.
Move SetAttributeNS onto JS<Element>.
Reimplement SetAttribute.
Reimplement SetAttributeNS.
Merge Attr::new_ns and Attr::new.
auto merge of #2060 : Ms2ger/servo/attributes-rewrite, r=jdm

There's more that I'd like to do, but this can land already.