Support [*|attr], attribute selectors in any namespace (fixes #1558)
auto merge of #3584 : ttaubert/servo/issue/1558-attr-ns-selectors, r=Manishearth

This implements basic support for attribute selectors with namespace prefixes. I would have added a more sophisticated test covering various selectors but it seems that we don't have an XML parser yet and thus no XHTML support?

r? @SimonSapin