Change Namespace::from_str to take &str, fix #1367
Move script::dom::namespace into util, in order to use it from style later.
Make get_attr() take a &Namespace rather than Option<~str>
Convert to Namespace during CSS parsing rather than selector matching.
Make Namespace::to_str() return a string.
Do not copy the result of get_link()
auto merge of #1561 : SimonSapin/servo/refactor-namespace, r=pcwalton

This builds on top of #1560 (… which I realize now may confuse Critic)

This refactors Namespace to reduce the overall amount of copying and conversion. In particular, it makes parsed selectors contain Namespace enums rather than strings.