add namespaces to elements
auto merge of #1358 : therealglazou/servo/therealglazou/namespaces, r=jdm

Add a namespace to elements. Sets by default the namespace to HTML for elements created by hubbub. Fixes :nth-of-type() and friends to match the namespace.

Note: prefix is not added by this PR because hubbub does not preserve it and I wanted to keep the patch as simple as possible.