Make Attr::value private.
Simplify Element::set_attribute for its remaining callers.
Implement support for parsed attributes.

This commit is heavily based on earlier work by Bruno Abinader in #2073.
Implement Element.classList (partially fixes #1717).
Implement parsed 'unsigned long' attributes.

This commit is partially based on earlier work by Bruno Abinader in #2073.
auto merge of #2616 : Ms2ger/servo/attrvalue, r=jdm

Obsoletes #2073.